An ELT Glossary : Interrogative words

Interrogative words (often called Wh- words) are words like What, Why, Where, When, How, Who, Whose  which can introduce a question. They may be :

a) Interrogative pronouns - ie they act as subject or object in the question, "replacing" the word which is subject or object in the answer.  Eg :

Subject : Who broke that vase?  / John broke it. 
Object : What did you see at the zoo? / We saw some tigers

b) Interrogative determiners, as part of a noun phrase. Again, these can be both subject or object of the verb. Eg  :

Subject : Which film won the Best Picture award?  /  Birdman won it
Object : Whose car did you borrow? / I borrowed Helen's car

c) Interrogative adverbs - ie they "replace" the adverbial in the answer. Eg :

Where did you go? We went to the museum.
When did you see her? / I saw her on Tuesday.
How did you travel? / We went by car.
Why did you do it? / I did it for the money.

How can also premodify other adverbs (How quickly can you do it? / How soon can you come?)  adjectives (How bad is it?) and determiners (How many were there?