Interrogative pronouns in English

Function

Interrogatives function both as pronouns:
Whose are these?
What are you going to buy?
and as determiners:
Whose coats are these? What coats are going to buy?

Who and whom

Whom is the form of the objective that is sued in formal spoken and written language. In colloquial English, the form 'who' is preferred:
Whom did you meet? (formal English)
Who did you meet? (informal English>

In formal English 'whom' is used after preposition. In colloquial English , the form 'who' with the preposition moved to the end of the sentence is preferred:
To whom did you speak? (formal English)
Who did you speak to? (informal English

What

'What' is used in the following situations:
1. As a pronoun:
a. In reference to non-personal objects:
What is your name?
What are you going to do tomorrow?
When 'what' is used with prepositions, the preposition is put at the end:
What are you fond of?
b. To denote profession:
What are you? I am a teacher.
2. As a determiner:
a. In reference to persons:
What girl is that?
it is more common to use 'who' in such context:
Who is that girl?
b. In sentences concerning size:
What length is that? where the verb to be must be used.

Which instead of who or what

'Who' and 'what' are used to denote a choice from an indefinite number of people or things. On the contrary, the use of 'which' implies that the choice is made from a limited number of people or things:
Who / What do you want to see? (general enquiry)
Which girls / rooms do you want to see? (limited choice)
It is also possible to say:
Which (of the) girls / rooms do you want to see?

The form of certain interrogative sentences

The verb is used in the affirmative when the interrogative pronoun is the subject of a sentence or when the interrogative modifies the subject of a sentence:
Who made that soup?
Which student got the award?