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?)