Relative pronouns introduce relative clauses that clarify or specify the antecedent. For example, in "Trees, which are plants, need sunlight to grow," the word "which" is a relative pronoun.

Relative pronouns tie in relative clauses that clarify or specify the subject.

For example:

He who laughs last is the slowest to get the joke.

In the preceding sentence, "who" is a relative pronoun that introduces the relative clause "who laughs last"—which helps narrow down and define the antecedent noun phrase "He".

The main relative pronouns are:

  • That
  • Which / Whichever
  • Who / Whom / Whoever / Whomever
  • Whose

But When, Where, and Why may also be called relative pronouns.
And What can be one too.

Additional References:

  1. The Wikipedia entry for Relative pronoun
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