Практическая грамматика

§ 8. Interrogative pronouns.

1. 
   Interrogative pronouns are used in inquiry, to form special questions. They are: who, whose, what, which.
   

The interrogative pronoun who has the category of case; the nominative case is who, the objective case whom.
Who refers to human beings?
Slipping her hand under his arm, she said: "Who was that?" "He picked up my handkerchief. We talked about pictures." (Galsworthy)
What when not attributive usually refers to things but it may be applied to persons when one inquires about their occupation.
"What are you looking for, Tess?" the doctor called. "Hairpins," she replied. (London) "What was he?" "A painter." (Galsworthy)
Which has a selective meaning: it corresponds to the Russian “который из” (an individual of the group). It may refer to persons and things.
The boys clasped each other suddenly in an agony of fright. "Which of us does he mean?" gasped Huckleberry. (Twain)
Which side of the bed do you like, Mum? (Galsworthy)
The questions Who is he? What is he? Which is he? differ in their meaning. The first question inquires about the name or par­entage of some person. The second question inquires about the occupation of the person spoken about. The third question inquires about some particular person out of a definite group of persons.
2. In the sentence interrogative pronouns may have different functions—those of subject, predicative, object and attribute:
Who, do you think, has been to see you, Dad? She couldn't wait! Guess. (Galsworthy) (subject)
"What's been happening, then?" he said sharply. (Eliot) (subject)
"No, who's he?" "Oh, he's a Polish Jew." (Aldington) (predica­tive)
"What are you, Mr. Mont, if I may ask?" "I, sir? I was going to be a painter." (Galsworthy) (predicative)
"What was her father?" "Heron was his name, a Professor, so they tell me." (Galsworthy) (predicative)
"He says he's married," said Winifred. "Whom to, for goodness' sake?" (Galsworthy) (object)
"Who do you mean?" I said. (Du Maurier) (object)
"What did you see in Clensofantrim?" "Nothing but beauty, dar­ling." (Galsworthy) (object)
"What sort of a quarrel?" he heard Fleur say. (Galsworthy) (ATTRIBUTE)
Whose pain can have been like mine? Whose injury is like mine? (Eliot) (attribute) Which day is it that Dorloote Mill is to be sold? (Eliot) (attribute)