Question words before to-infinitives are used when a speaker wants to express a choice, decision, method, place, time, person, or action without using a full subordinate clause. This pattern often appears after verbs related to knowing, deciding, asking, explaining, or learning.
A question word such as what, where, when, who, which, or how is followed by to + the base form of a verb. Learners also need to know when this pattern can replace a longer clause.
What are question words before to-infinitives?
Question words before to-infinitives form a phrase in which a question word comes before a to-infinitive. The phrase usually expresses information that is unknown, undecided, or needed.
The basic structure is:
Subject + main verb + question word + to + base verb
Example: I do not know what to say.
Here, what to say is close in meaning to what I should say. It commonly follows verbs such as know, decide, ask, tell, show, learn, wonder, and explain.
Examples:
- We have not decided where to meet.
- Can you show me how to use this program?
Which question words can come before a to-infinitive?
Choose the question word according to whether the sentence refers to a thing, choice, place, time, person, or method.
What and which
Use what + to-infinitive when the action or information is not yet identified. Use which + to-infinitive when choosing from a known or limited set of options.
Examples:
- I am not sure what to write.
- Do you know which bus to take?
A noun can also follow what or which.
Example: Please tell me what documents to bring.
Where and when
Use where + to-infinitive for a place and when + to-infinitive for a suitable or necessary time.
Examples:
- We need to find out where to park.
- He has not decided when to leave.
Who and whom
Use who + to-infinitive when the missing information is a person connected with the action. In formal writing, whom can sometimes be used as an object, but who is more common in everyday English.
Example: I do not know who to ask.
How
Use how + to-infinitive to express the method or way of doing something.
Examples:
- She learned how to create a presentation.
- Could you explain how to solve this problem?
Whether
Although whether is not a wh-question word, it commonly appears in the same pattern. Whether + to-infinitive expresses uncertainty between alternatives.
Example: We cannot decide whether to stay or leave.
Do not normally use if directly before a to-infinitive.
Example: Incorrect: I do not know if to accept the offer. → Correct: I do not know whether to accept the offer.
What about why?
Why does not normally appear directly before a to-infinitive in this pattern. When the reason is unknown, English usually keeps a full clause.
Example: Incorrect: I do not know why to leave. → Correct: I do not know why I should leave.
How to use question words before to-infinitives
This structure is useful when the person performing the infinitive action is already clear from the context. It keeps the sentence concise while preserving the intended meaning.
After verbs of knowledge and uncertainty
Verbs such as know, remember, wonder, and find out can introduce this pattern.
Examples:
- I do not know how to answer this question.
- We need to find out who to call.
After verbs of choice and decision
Use the pattern after verbs such as decide, choose, and consider when the sentence focuses on a decision.
Examples:
- They have not decided where to stay.
- He is considering whether to apply.
After verbs of instruction and communication
The structure also appears after tell, show, teach, ask, and explain.
Examples:
- Please tell me what to prepare.
- She showed us how to open the file.
With explain, use to before the person when necessary.
Example: Incorrect: She explained me how to use it. → Correct: She explained to me how to use it.
How to shorten a clause with question word + to-infinitive
A longer clause can sometimes be reduced to question word + to-infinitive when the intended subject of the infinitive is clear. The shorter version often corresponds to a clause containing should, can, or could.
A common pattern is:
Question word + subject + should/can/could + base verb → question word + to + base verb
Examples:
- I do not know what I should do. → I do not know what to do.
- Please tell me where I can buy a ticket. → Please tell me where to buy a ticket.
- She wondered how she could solve the problem. → She wondered how to solve the problem.
If removing the subject creates ambiguity or changes the meaning, keep the full clause.
Question word + to-infinitive vs. similar structures
These forms may look similar, but their grammar is different. The table below shows the main distinctions.
| Structure | Key point | Example |
| Question word + to-infinitive | A compact phrase about what, where, when, who, which, or how to act | I do not know what to do. |
| Direct question | A complete question, often with inversion | What should I do? |
| Indirect question | A subordinate clause with statement word order | I do not know what I should do. |
| Whether + to-infinitive | Expresses uncertainty between alternatives | She cannot decide whether to go. |
| Why + full clause | Usually replaces why + to-infinitive | I do not know why I should go. |
A key rule is that indirect questions use statement word order, not direct-question order.
Example: Incorrect: I do not know what should I do. → Correct: I do not know what I should do.
Common mistakes with question words before to-infinitives
- Using the -ing form after the question word: The pattern requires to + base verb. Example: Incorrect: Do you know how solving this problem? → Correct: Do you know how to solve this problem?
- Using why directly before to + verb: Keep a full clause when asking about a reason. Example: Incorrect: She does not know why to change it. → Correct: She does not know why she should change it.
- Using if + to-infinitive: Use whether for alternatives. Example: Incorrect: We cannot decide if to wait. → Correct: We cannot decide whether to wait.
- Choosing the wrong question word: Match it to the information needed. Example: Incorrect: I do not know when to put the box. → Correct: I do not know where to put the box.
- Keeping question order in an indirect clause: Use statement order. Example: Incorrect: Can you tell me where can I buy it? → Correct: Can you tell me where I can buy it?
Practice exercises on question words before to-infinitives
Use the exercises below to check the structure in context.
Complete each sentence with what, where, when, who, how, or whether.
- I cannot decide ______ to wear to the school event.
- Can you show me ______ to upload this file?
- We have not decided ______ to meet after class.
- She does not know ______ to ask for advice.
- He is wondering ______ to accept the invitation.
Rewrite each sentence using question word + to-infinitive.
- I do not know what I should say.
- Please tell me where I can find the information.
Answers: 1. what; 2. how; 3. where; 4. who; 5. whether; 6. I do not know what to say.; 7. Please tell me where to find the information.
Question words before to-infinitives help express choices, decisions, instructions, and uncertainty in a concise form. Remember the core pattern question word + to + base verb, choose the question word according to the information needed, and avoid errors such as why + to-infinitive or if + to-infinitive.
See more:
- The Suggest Structure in English: Rules, Usage, and Examples
- English Vowels Explained: Definition, Types, and Correct Pronunciation
Frequently asked questions
Can all question words be used before a to-infinitive?
No. Common choices include what, which, where, when, who, and how, while why usually requires a full clause.
What is the difference between what to do and how to do it?
What to do identifies the action that should be taken, while how to do it explains the method used to perform that action.
Can whether be used before a to-infinitive?
Yes. Whether + to-infinitive is used when someone is uncertain about alternatives, as in I do not know whether to stay.
