‘What’ and ‘which’ are interrogative pronouns. This means they stand for something the speaker doesn’t yet know. They are used for objects, qualities, or places, but not for people. For people, we use the word ‘who’.

The difference between ‘what’ and ‘which’ are how many possibilities there are and how much you know about those possibilities.

See Unit 12.

‘what’ is for lots of possibilities

We use ‘what’ when there are lots of possible options or when we don’t know how many options there are. Here are some examples of using ‘what’ in this way:

What color shirt will you wear tomorrow?
What is your favorite kind of ice cream?
I didn’t hear what you said.
What did you do today?
Do you know what time it is?

‘which’ is for less possibilities

‘which’ indicates there are fewer possible options. We already know that the answer is one of two things or one of a few things. Knowing how to use ‘which’ in a question means thinking about how much you already know. Using it indicates you already have some information, as you can see in these examples:

Which pair of shoes did you decide to wear? (I know what shoes you own)
Which of your children is starting school this year? (I know you have more than one child)
Which arm did you hurt? (you only have two)
Do you know which way we should turn? (it can only be left or right)
Which day did you decide to have the party? (there are only seven days in a week)

sometimes either one is okay

There are some situations when either ‘what’ or ‘which’ can be used. These are times when there are several options but not unlimited choices. Here are a couple of examples:

What flight are you taking to London?
Which flight are you taking to London?
(there are many flights leaving the airport, but it’s a limited number)

What day is your surgery?
Which day is your surgery?
(there are a limited number of days in a week or month)