We use interrogative pronouns to ask questions. The interrogative pronoun represents the thing that we don't know (what we are asking the question about).

There are four main interrogative pronouns: who, whom, what, which

Notice that the possessive pronoun whose  can also be an interrogative pronoun (an interrogative possessive pronoun).

 subjectobject
personwhowhom
thingwhat
person/ thingwhich
personwhose
Notice that whom  is the correct form when the pronoun is the object of the verb, as in "Whom  did you see?" ("I saw John .") However, in normal, spoken English we rarely use whom . Most native speakers would say (or even write): "Who  did you see?"

Look at these example questions. In the sample answers, the noun phrase that the interrogative pronoun represents is shown in bold.

questionanswer 
Who  told you?John  told me.subject
Whom  did you tell?I told Mary.object
What 's happened?An accident's happened.subject
What  do you want?I want coffee.object
Which  came first?The Porsche 911 came first.subject
Which  will the doctor see first?The doctor will see the patient in blue first.object
There's one car missing. Whose  hasn't arrived?John's (car) hasn't arrived.subject
We've found everyone's keys. Whose  did you find?I found John's (keys).object

Note that we sometimes use the suffix "-ever" to make compounds from some of these pronouns (mainly whoever, whatever, whichever ). When we add "-ever", we use it for emphasis, often to show confusion or surprise. Look at these examples:

  • Whoever ¬†would want to do such a nasty thing?
  • Whatever ¬†did he say to make her cry like that?
  • They're all fantastic!¬†Whichever ¬†will you choose?

sources : Original Link