The interrogative pronouns are: who, whom, whose, what and which. They have nominal functions:
Who opened my letter? (who: subject; opened my letter: predicate)
When the wh-element is the subject it is immediately followed by the finite or conjugated verb, and there is no inversion of order.
Who (or whom) did you see? (you: subject; who did see: predicate; who/whom: direct object)
3)Indirect object (introduced by the preposition to):
Who did you give the present to? (you: subject; who to: indirect object)
4)Objet to the preposition:
With whom did you go? / Who did you go with? (with: preposition; whom/who: object to the preposition)
Whose is that book? (that book: subject; whose: subjective complement)
What did they make him? (what: objective complement; him: direct object)
The interrogative adjectives are: whose, what and which. They have an adjectival function.
Which books have you lent him? (you: subject; which books: direct object)
The direct object is a noun phrase (which books). Within the noun phrase, ‘books’ functions as the Head (the most important word) and ‘which’ pre-modifies books. ‘Which’ is an interrogative adjective.
Whose antiques are these? (these: subject; whose antiques: subjective complement, noun phrase)
Which (pronoun and adjective) is used instead of ‘who and what’ when the choice is restricted.
What will you have to drink? (general inquiry)
There’s whisky, gin and sherry; which will you have? (restricted choice)
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