Learn English – What vs. Which – correct usage in questions

questionsusage

Today I found myself in a situation that I asked "Which class" to a friend who said that "we are going to the class". Then one of my friends said that I should ask "what class" and another one said both (what class and which class) are correct. Who is right?

This issue I meet often when I'm talking about things, such as:

What day/ which day?

What floor / which floor?

what teacher / which teacher?

What page / which page?

What place / which place?

What line (bus) / Which line?

What /which sentence (do you agree with)?

Best Answer

This is actually a rather simple answer although it's rarely explained in simple terms:

What seeks the answer from an undefined set while which asks you to choose among possible options. (Additionally, you could ask which when discussing people.)

For instance, to use your first example, let's say that I was making an appointment with a friend:

"What day are we meeting?"
"I'm available on Tuesdays and Thursdays."
"Which one works best for you?"
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