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I've always thought that I knew exactly what the distinction is between 'what' and 'which' until a couple of days ago when I came across a classroom situation which completely confused me. It was a conversation in an EFL textbook. Here is the part which confused me:

A asked B "Which building is it?"

B replied "It is a hospital."

Can anyone tell me whether or not this is correct? Because if I was to ask about the same thing, I'd say 'What building is it?' And I'd only use 'Which building is it?' when I'm looking for a particular building, a department store for instance, and I've got so many buildings in front of me that I don't know which one it is. So I'd ask someone 'Excuse me. I'm looking for the department store. Which building is it?' And I'd expect an answer like 'It's the third building on the left.' instead of answers like 'It is a/an ________.' Could someone please explain this to me? Thank you!!
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The dialog you quote doesn't provide enough context for a definitive answer to your question.

In general however, in the scenario you outline, either word could be used.

"Which/What building is it?" Could elicit either of the following answers, depending on the situation and the preference of your interlocutor.

"It's the third one on the left."

"It's the hospital."
chenyinchengAnd I'd expect an answer like 'It's the third building on the left.' instead of answers like 'It is a/an ________.'
Me too. Personally, I don't care for that little exchange in the EFL book either. Nevertheless, in that situation what and whichtend to be used somewhat interchangeably by many speakers, so, at least in that sense, the exchange is "authentic English". Emotion: smile

CJ
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Sorry, but I still don't understand how this is interchangeable. I read a question posted by another user asking about the usage of "which" and "what", too. And you kindly replied, "which" chooses. "what" identifies. In this sense, choices were implied when one asked "Which building is it?". Then how could the reply be "It is a hospital.", as this identifies the building?

Hi chenyincheng,

Which- when you ask "which", in your own mind you have narrowed down the choices to a few from which you expect the final answer.

What - When you ask "what", you have no particular clue as to what to expect.

So for a broarder question, "what" will be sort of an universal question to extract the answer. In the following, Susan may use Which and what in her inquiry.

Mary: John looked extremely sharp in his suit today.

1) Susan: which suit did he wear, the dark blue pin-tripe, or the Beige?

2) What suit was he wearing?

In other words, which is more specific than what.
chenyinchenghow this is interchangeable
"tends to be used interchangeably" does not mean "is interchangeable". I'm saying that your analysis is correct, but that you should not be surprised to hear either "which" or "what" in many situations.

CJ
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CalifJim
chenyinchenghow this is interchangeable
"tends to be used interchangeably" does not mean "is interchangeable". I'm saying that your analysis is correct, but that you should not be surprised to hear either "which" or "what" in many situations.

CJ

I see. Thank you very much!!

You're right!Emotion: smile