Meaning and use
A simple way of asking questions in English is by using interrogatives, or question words, such as who or what. These questions are called wh-questions and are used when asking for information.
What time is it?
Who ate the biscuits?
Look at this sentence:
Sally met David Beckham.
We can ask about the subject or object of this sentence:
Asking about the subject: Who met David Beckham? Sally met David Beckham.
Asking about the object: Who did Sally meet? Sally met David Beckham.
The first question is a subject question because who refers to the subject. There is no auxiliary. The second question is an object question because who refers to the object and comes before the auxiliary did.
Subject questions with no auxiliary are formed with: question word + verb + object, where the verb agrees with the subject.
‘Who speaks Japanese?’ ‘Kento speaks Japanese.’
‘Who rang the doorbell?’ ‘The milkman rang the doorbell.’
‘What caused the accident?’ ‘Bad weather caused the accident.’
Whose and which ask about possession and choice,and can be used in subject questions like this:
Whose horse finished the race first?
Which painting cost the most?
Take note: using ‘what’ or ‘which’
As well as which, what is also used to ask about choices. If the choice is limited, we use which and this is usually followed by a noun.
What social networks do you use?
What happenedto your shirt?
Which chair is yours?
Which of these restaurants has the best service?
Which hand do you write with?
In casual speech, who is or who has often becomes who’s. This can cause problems for the listener because it sounds the same as the question word whose, which is used to show possession. Whose usually comes before a noun.
Who’s coming for dinner?
Whose bike got stolen?
Whose hat cost over £100?
Whose mobile phone still has a signal?
Reference: BBC Learning English