Subject + verb + (conjunctive + infinitive)

A conjunctive is an interrogative word which also serves as a conjunction. Examples are: where, what, when and how.

A conjunctive followed by an infinitive can be used as the direct object of a verb.

Read the following sentences:

I don’t know what to do with this.
I don’t know where to turn for help.
We must find out what to do next.
Have you been told when to start?
Do you know how to stop it?
We should inquire how to reach the place.
We must remember where to turn off the main road.

Notes:
A ‘conjunctive + infinitive’ alone cannot be used as a question. For example, it is wrong to use ‘what to do?’ as a question. You must say ‘what shall we do?’ or ‘what is to be done?’

The conjunctive + infinitive can be changed into a noun clause:

Compare:

For example, the sentence ‘I don’t know what to do with this’ means the same as ‘I don’t know what I should do with this’.
Similarly, ‘We must find out what to do next‘ means ‘We must find out what we should do next’.

Subject + verb + indirect object + (conjunctive + infinitive) as direct object

I will show you how to manage it.
Could you tell me where to find a good hotel?
Show him where to put his things.
Could you tell me how to start the washing machine?
Someone should teach you how to behave.
A dictionary should tell you how to pronounce the words.

Here also the conjunctive + infinitive can be changed into a noun clause.

I will show you how you can manage it.
Could you tell me where I can find a good hotel?