The interrogative pronouns are words like who, which, what etc. As their name suggest, they are used to introduce questions.

• What is this?
• Who are you?
• Which do you prefer?

Which and what

In most cases, both which and what are possible with little difference of meaning.

• What language do they speak in Brazil? OR Which language do they speak in Brazil?

Which is preferred in cases where the speaker is thinking about a limited number of choices. What is used in cases where the speaker is thinking about an unlimited number of choices.

• Which is your favorite color? (The number of colors are somewhat limited.)
• What is your favorite number? (Here the number of choices in unlimited.)

The interrogative pronouns also act as determiners.

• Which writer do you like best? (Here the interrogative pronoun which acts like an adjective modifying the noun writer.)

When used like this they are sometimes called interrogative adjectives.

Before a pronoun or a noun with a determiner (e.g. the, my this), we use which of. What and who are not used with of.

• Which of these shirts are yours? (NOT What of these shirts are yours?)

When interrogative pronouns are used to introduce adjective clauses, they are called relative pronouns.

• I know a girl who writes good stories. (Here the interrogative who is used to introduce the relative clause ‘who writes good stories’.)

The interrogative pronouns can also be used to introduce noun clauses.

• I don’t know who did this.

Here the interrogative who introduces the noun clause who did this.