A) Formation: interrogative pronouns or interrogative adverbs such as what, which, how, when, where, followed by infinitive verbs or infinitive phrases.

B) Function: According to the sentence components, it can generally be used as subject, object and predication in a sentence, such as how to save children is the most important thing for us.

I don't know what to do next. (as an object)

The question is when to start.

The structure of "interrogative word+infinitive verb" can be rewritten as a clause guided by interrogative words, and the sentence components of the clause remain unchanged, such as "I don't know what to do". I don't know what I should do.

postscript

Interrogative pronouns and interrogative adverbs have different meanings. As long as you know which are interrogative pronouns and which are interrogative adverbs, you can distinguish them:

1. Interrogative pronouns can be used as subject, object, predicative and attribute in interrogative sentences.

Common ones are: what, who, who, which, etc.

For example: What color is it? It is red. ?

2. Definition of interrogative adverbs Adverbs used to ask questions such as time, place, modality, method, degree, reason and reason are called interrogative adverbs. There is no difference between general interrogative adverbs and special interrogative adverbs.

Adverbs with questions, how, where, why and how.

When was the last time you saw her? When was the last time you saw her? )?

Where does she live? Where does she live? )?

How can I let you know the result? How can I inform you of the result? )?

Why did you leave the door open? Why did you leave the door open? )?

Question adverbs are generally used at the beginning of sentences, and sentence patterns are in the form of questions (as shown in various examples)?

The interrogative sentences guided by interrogative adverbs can also be used indirectly in sentences to become indirect interrogative sentences. At this time, we should pay attention to the changes in the position of the subject and the changes in predicate verbs.

When was the last time you saw her? (direct question)?

Tell me when was the last time you saw her. (Indirect question) Tell me, when was the last time you saw her?

Where does she live?

Do you know where she lives? Do you know where she lives? )