Difference between what and which.

The words 'what' and 'which' have similar uses and meanings. Both are used to obtain information about a thing or person. However, there are some differences between them.

What

It expresses a general meaning. It is used when talking about an unlimited or indefinite number of options, answers, or possibilities.

・What is your name?

・What are you doing?

・What colour are your eyes?

・I don't understand what you mean.

Which

It expresses a narrower meaning compared to 'what'. It is used when talking about a limited or specific number of options, answers, or possibilities.

・Which shirt should I wear to the party?

・Which book did you borrow from the library?

・I don't know which song is playing.

・Which of these options do you prefer?

・Which do you prefer? Coffee or tea?

'What' vs 'Which'

Both what and which can be used when asking a question about a person or thing. If there is an unlimited or indefinite number of answers to the question, 'what' is used; and if there is a limited or certain number of answers, 'which' is used.

・What is your favorite colour?

・Which is your favorite colour? Green or blue?

Let's say the speaker and the listener want to watch a movie. If the speaker has no movie in mind, he uses 'what'. If the speaker has suggested several movies to the listener and is undecided among these suggested movies, he uses 'which'.

・What movie shall we watch?

・Which movie shall we watch?

Let's say the speaker sees the listener reading a book in the park. The speaker uses 'what' if he has no idea which book the listener is reading. If the speaker knows that the listener is currently reading some author's book, but does not know which one, he uses 'which'.

・What book are you reading?

・Which book are you reading?