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Infinitives after question words: who to …, etc

Indirect questions: Tell us what to do.​

In indirect speech (see here), we can use an infinitive after the question words who, what, where, etc (but not usually why). This structure expresses ideas such as obligation and possibility.

  • I wonder who to invite. (= … who I should invite.)
  • Tell us what to do.
  • Can you show me how to get to the station? (= … how I can get to the station?)
  • I don’t know where to put the car.
  • Tell me when to pay.
  • I can’t decide whether to answer her email.
  • (but not I can’t understand why to do it.)

Direct questions: What shall we do?​

We do not usually begin a direct question with How to …?, What to …?, etc. After question words, we often use shall and should.

  • How shall I tell her? (not How to tell her?)
  • What shall we do? (not What to do?)
  • Who should I pay? (not Who to pay?)

Titles: What to do if fire breaks out.​

How to …, What to …, etc are often found as titles for instructions, information leaflets, books, etc. (Note: these are not questions.)

  • HOW TO IMPROVE YOUR PRONUNCIATION
  • WHAT TO DO IF FIRE BREAKS OUT
note

For questions beginning Why (not) + infinitive, (see here).