Relative clauses begin with a relative pronoun:
who or that for people and animals (this is less common but we still use it, especially in speech).
which or that for things (that is common in speech).
whom for people as the object of the relative clause.
Whom is the object form of who. It is used in formal styles or in writing. There has been a steady decline in its use. It is natural in informal English to use who instead of whom, even when it is the object of the relative clause.
- Whom did you meet at the party? (It is becoming unusual in both spoken and written English.)
- Who did you meet at the party? (more common)
whose for possession.
It has to be used after a preposition.
- Do you know to whom the report was addressed?
- Do you know who the report was addressed to? (It is more usual to put the preposition at the end of the clause and use who.)
We can also use when, where, and why as relative pronouns to refer to a time, a place, or a reason.
→ Where can be replaced by which/that + preposition.
- The hotel where we stayed was wonderful. or
- The hotel in which we stayed was wonderful. (formal) or
- The hotel that/which we stayed at was wonderful.
→ We can do the same with when.
- Do you know the year when the first personal computer was made?
- Do you know the year in which the first personal computer was made?
The relative pronoun can be:
- the subject of the clause: we cannot leave out the relative pronoun
- He is the man (subject) who gave us a lift.
- the object of the clause: we can leave out the relative pronoun
- He is the man (object) (who) we asked for help.
= subject is we so we can leave out who