Depending on what sort of information we want, we use one of the following question words:
Who wrote that book? (person)
What is your name? (thing)
Which book is yours? (thing)
When did you arrive? (time)
Where do you live? (place)
Why are you doing that? (reason)
How can I find out? (manner)
What and which can often be used with the same meaning. When the person asking the question has a restricted number of choices in mind, s/he will use which. When s/he is not thinking of a restricted number of choices, what is used:
Which main course (from the menu) are you going to have?
Which department (of this company) do you work in?
What name is on the envelope?
What number shall I call?
Whom is a more formal way of saying who, and is not common when speaking. If we choose to put our question word after a preposition, then we must use whom:
With whom did you go?
However, this is very unusual, and we would normally avoid this by putting the preposition at the end of the phrase:
Who did you go with?
Apart from these single words, we combine two or more words to find out other kinds of information:
How old are you?
What time is it?
How many children have you got?
How long did it take?
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