Wh-Word + To-Infinitive Clauses in English: Why “I Don’t Know What to Do” Is a Complete Structure

A student says: > I don’t know what to do. The sentence is complete. But the phrase: > what to do looks strange if we expect every question to contain a finite verb, a visible subject and ordinary question word order. Where is: > should I? A fuller paraphrase might be: > I don’t know what I should do. English can compress that indirect question into: > what to do. The result is a **wh-word + to-infinitive clause**. Examples include: > what to say > where to go > how to begin > when to leave > who to ask > which to choose > whether to continue. These structures are common because they package a decision, method, destination or unresolved choice without repeating a full finite clause. The reader’s job is: > recover the missing relationship without inventing the wrong subject. ## Quick answer: what is a wh-word + to-infinitive clause? A wh-infinitive combines an interrogative expression with a non-finite to-infinitive. Example: > We don’t know where to wait. Approximate fuller meaning: > We don’t know where we should wait. Another: > She showed me how to open the file. Approximate meaning: > She showed me how I should/could open the file. Another: > They have not decided when to leave. Approximate meaning: > They have not decided when they should/will leave. The exact modal paraphrase changes with context. The structure itself remains: > interrogative meaning + non-finite clause. ## It is an indirect question, not a direct question Direct: > What should I do? Embedded finite question: > I don’t know what I should do. Wh-infinitive: > I don’t know what to do. The last two are embedded inside a larger clause. That means they do not use direct-question inversion: > *I don’t know what should I do. Standard English uses: > I don’t know what I should do. Or the non-finite version: > I don’t know what to do. This connects naturally to eduKateSG’s existing article on embedded questions. ## “What” still has a role inside the infinitive Take: > what to read. The infinitive: > to read needs an object. What will be read? > what. So the wh-word is not a decorative question label. It participates in the grammar of the infinitive. Likewise: > who to invite means: > invite who? And: > which route to take means: > take which route? The wh-element fills a semantic/syntactic slot associated with the infinitive. ## “Where” asks for location > I don’t know where to sit. The unresolved information is: > location. A fuller paraphrase: > I don’t know where I should sit. Now: > She showed us where to place the equipment. The phrase gives the correct location. The clause can therefore appear after verbs of knowing, deciding, showing, explaining, remembering and asking. ## “How” asks for method > He explained how to reset the router. The unresolved content is: > method. Now compare: > He explained that the router needed resetting. That-clause: > proposition. Wh-infinitive: > procedure/method. English can package different knowledge types through different complement structures. ## “When” asks for time or timing > We haven’t decided when to leave. Possible fuller version: > when we should leave. The question is not: > whether departure will happen. It is: > at what time. Wh-word choice identifies the unresolved variable. ## “Who to ask” is about the object role > I don’t know who to ask. A common interpretation is: > I don’t know who I should ask. The understood subject is: > I. The wh-word: > who is the person receiving the question/request. This can be tricky because **who** appears at the front but is not the subject of: > ask. Word order hides the internal role. ## “Who to invite” works the same way > Mei hasn’t decided who to invite. Fuller: > Mei hasn’t decided who she should invite. Subject of inviting: > Mei. Object: > who. This is a useful reminder: > front position does not automatically mean subject. Grammar must be reconstructed. ## The subject is often understood Non-finite clauses commonly lack an overt subject. Sentence: > Amir knows what to do. Who does the doing? Most naturally: > Amir. The main-clause subject controls the understood subject of the infinitive. Paraphrase: > Amir knows what Amir should do. English avoids repeating the name. ## But the understood subject can come from an object Consider: > The teacher showed Lina where to sit. Who should sit? > Lina. Not necessarily: > the teacher. The object: > Lina supplies the logical subject of: > to sit. This is why interpretation cannot be reduced to: > “the main subject always does the infinitive.” The higher verb matters. ## “Tell” makes the object relationship visible > The teacher told Amir where to wait. Who waits? > Amir. Fuller: > The teacher told Amir where Amir should wait. The object of: > told is the understood subject of the infinitive. This is a productive pattern in instructions. ## Context can supply the subject > The sign explains where to park. Who parks? Not: > the sign. The understood subject is generic: > drivers / people using the car park. Context supplies the actor. This is common in instructions, manuals, notices and guides. Non-finite clauses can therefore have an implicit general subject. ## “Whether to” handles yes/no choice A direct yes/no question: > Should we leave? Embedded finite: > We don’t know whether we should leave. Non-finite: > We don’t know whether to leave. The word: > whether introduces an unresolved binary choice. Examples: > whether to stay > whether to apply > whether to continue > whether to tell the teacher. This pattern is extremely common in decision language. ## Use “whether”, not “if”, before the infinitive Natural: > I don’t know whether to go. Not standard: > *I don’t know if to go. English uses: > whether + to-infinitive. This is one of the clearest form rules in the construction. Students who know: > if / whether as near-alternatives in finite clauses need to learn that they are not interchangeable everywhere. ## Why is different Many learner grammars note that ordinary English does not normally use: > why + to-infinitive in the same productive way. Instead of: > *I don’t know why to leave, English normally says: > I don’t know why I should leave. Or: > I don’t know why I need to leave. The reason variable behaves differently from: > what / where / when / how / who / which. This is an important boundary. ## “Why not…?” is a different construction English can say: > Why not leave now? That is a direct suggestion/question structure. It does not prove that ordinary embedded: > why to leave is productive. Do not transfer one construction into another. Surface words can resemble each other while grammar differs. ## “Which to choose” > I don’t know which to choose. The comparison set must be contextually clear. Maybe two books are visible. Maybe several plans have already been discussed. The word: > which selects from an established set. Now: > I don’t know what to choose. The set is less explicitly bounded. This is the same semantic difference seen in finite questions. ## “Which route to take” A wh-phrase can contain a noun: > which route to take. Here: > which route is the object of: > take. The infinitive clause can therefore begin with more than a single wh-word. Examples: > which book to read > what colour to choose > how many copies to print. The interrogative phrase can carry internal structure. ## “How many to buy” > I’m not sure how many to buy. Recover: > how many [items] I should buy. The noun may be omitted because the context already supplies it. This means wh-infinitives can combine with ellipsis inside the interrogative phrase. English compresses at several levels at once. ## Wh-infinitives are common after “know” > know what to do > know where to go > know how to begin. These combinations are common because: > know can take interrogative complements. The phrase expresses procedural or decision knowledge rather than a simple fact. Compare: > I know that the door is locked. Propositional knowledge. > I know how to lock the door. Procedural knowledge. Different complement. Different knowledge type. ## “Learn how to” is procedural > Students learn how to calculate percentage change. The clause does not mean: > students learn a proposition. It means: > they acquire a method or skill. This is especially important in education. Language distinguishes: > knowing that > knowing whether > knowing how. Those are different epistemic structures. ## “Decide what to…” packages a plan > The committee decided what to include. The decision resolves: > selection. Now: > The committee decided to include the appendix. This is not a wh-infinitive. It directly states the chosen action. Compare: > decided what to include > decided to include X. One packages the choice problem. The other packages the chosen action. ## “Explain how to…” versus “explain what…” > The tutor explained how to factorise the expression. Method. > The tutor explained what factorisation means. Concept/content. Good teaching uses both concept and procedure. Students should recognise the grammar of each. ## A Singapore classroom example Student: > I know the formula, but I don’t know when to use it. This is a powerful diagnostic sentence. The learner has: > formula knowledge. The missing knowledge is: > condition for application. The wh-infinitive identifies the gap: > when. Now: > I don’t know how to use it. Different gap: > procedure. Now: > I don’t know which formula to use. Different gap: > selection. Grammar maps different kinds of uncertainty. ## Mathematics A student may ask: > How do I know which method to use? Inside that question is another wh-infinitive: > which method to use. The challenge is not computation. It is method selection. This is exactly why wh-infinitives are useful in Mathematics education. They package decision points. ## Science Laboratory guide: > The table shows how to calculate the rate. Procedure. > The safety notice explains where to dispose of the material. Location. > The protocol states when to stop heating. Timing. Wh-infinitives are common in procedural Science because procedures contain unresolved variables: – method; – place; – timing; – choice. ## Humanities A student planning an essay may know the historical facts but not: > what to argue > which evidence to select > where to place a counterargument. The grammar names the planning problem. It can therefore help teachers diagnose writing difficulty more precisely. ## Situational writing A student may write: > Please tell me where should I report tomorrow. This blends direct-question inversion into an embedded question. Correct finite: > Please tell me where I should report tomorrow. Or compact: > Please tell me where to report tomorrow. The second is concise and natural. ## “How to” is not always a question A title: > How to Write a Summary looks like a wh-infinitive. It functions as a procedural heading. The underlying idea is: > how one can/should write a summary. The construction has become a standard genre format for instructions and guides. This is why search pages, manuals and learning resources use: > how to… so frequently. ## “What to do” can become a noun-like unit in discourse > What to do next is unclear. The whole clause: > what to do next functions as subject of: > is unclear. This shows that the wh-infinitive is a clause that can occupy positions normally occupied by noun phrases. It packages an unresolved action-choice as a grammatical unit. ## A quiet literary lens A character may stand at a door thinking: > where to go > what to say > whether to knock. These compressed clauses fit moments of decision. They make the structure of uncertainty visible without explaining every thought. A controlled writer can use the grammar of choice to hold perspective close. The lesson is not stylistic imitation. It is: > grammar can encode the shape of a decision. ## Diagnosis before prescription ### Gap 1: direct-question inversion inserted Learner: > *I don’t know what should I do. **Repair:** use finite embedded order or wh-infinitive. ### Gap 2: understood subject assigned automatically to main subject **Repair:** test higher verb and any object. ### Gap 3: if used before to-infinitive Learner: > *if to go. **Repair:** use: > whether to go. ### Gap 4: why used productively like other wh-words Learner: > *why to leave. **Repair:** use a finite modal clause. ### Gap 5: wh-word role ignored Learner treats: > who to ask as if **who** were the asker. **Repair:** expand the full clause. ### Gap 6: procedural knowledge confused with propositional knowledge **Repair:** compare: > know that > know how to. ## A reconstruction method Sentence: > Lina knows where to wait. ### Step 1: identify wh-variable > where = location. ### Step 2: identify infinitive > to wait. ### Step 3: identify understood subject > Lina. ### Step 4: expand > Lina knows where Lina should wait. Now: > The teacher told Lina where to wait. Who waits? > Lina. The object supplies the understood subject. ## A six-pattern table | Form | Typical unresolved information | |—|—| | what to do | action/content | | how to do it | method | | where to go | place | | when to leave | time | | who to ask | person/object | | which to choose | selection | | whether to continue | yes/no decision | The table is a semantic map, not just a grammar list. ## Internal-link opportunities This article can connect to: – [Embedded Questions in English](https://edukatesg.com/2026/08/29/embedded-questions-indirect-question-word-order-english/) – [Gerund vs Infinitive Meaning Changes](https://edukatesg.com/2026/08/30/gerund-infinitive-meaning-change-stop-remember-try-english/) – [Object + Infinitive in English](https://edukatesg.com/2026/08/29/object-infinitive-believe-him-to-be-raising-english/) – [Verb Patterns in English](https://edukatesg.com/2026/08/28/verb-patterns-lexicogrammar/) – [Free and Fused Relative Clauses in English](https://edukatesg.com/2026/08/29/free-fused-relative-clauses-english-what-you-need/) – [How Language Works](https://edukatesg.com/2026/08/26/how-language-works/) The distinct reader intent is: > wh-word + to-infinitive → reconstruct indirect question → identify missing information type and understood subject ## Connections eduKateAI can learn **Wh-word ↔ uncertainty:** the interrogative element identifies which part of a possible action remains unresolved. **Infinitive ↔ non-finite clause:** the construction lacks ordinary finite tense/agreement and often has an understood subject. **Context ↔ subject:** the actor of the infinitive can be controlled by the main subject, a higher object or general discourse context. **Whether ↔ binary choice:** **whether to** packages a yes/no decision; **if to** is not the normal Standard English pattern. **How ↔ procedure:** wh-infinitives provide a compact grammar for methods and instructions. **Reading ↔ reconstruction:** expanding the clause into a finite indirect question helps expose hidden roles. **Subjects ↔ decision points:** Mathematics, Science and Humanities use wh-infinitives naturally to express method selection, timing, procedure and choice. **Writing ↔ compression:** wh-infinitives reduce repeated subjects and modal verbs while preserving interrogative meaning. **AI language understanding ↔ implicit arguments:** robust parsing requires recovering the understood subject and the grammatical role of the wh-expression. ## Final checkpoint Expand: > I don’t know what to do. A natural version is: > I don’t know what I should do. Now: > The teacher told Amir where to wait. Who waits? > Amir. And which is standard? > whether to leave > *if to leave If the learner can reconstruct those relations, wh-word + to-infinitive clauses have become visible English grammar. ## Research basis This article was informed by the public research pass, including: – Cambridge Grammar, **Infinitives with and without to**: https://dictionary.cambridge.org/grammar/british-grammar/infinitives – Huddleston, Pullum & Reynolds, **Non-Finite Clauses**, *A Student’s Introduction to English Grammar* (2022): https://www.cambridge.org/highereducation/books/a-students-introduction-to-english-grammar/EB0ABC6005935012E5270C8470B2B740/non-finite-clauses/8C8CCC879F04B8B2014746C2804A3CAA – RIT Supporting English Acquisition resources on logical subjects and word order in infinitival clauses; – reference-grammar work on interrogative infinitival clauses and wh-dependencies. The article uses fuller “should/can” paraphrases as teaching diagnostics, not as claims that every wh-infinitive has one identical hidden modal.