Relative Pronouns: Who, Which, and That

A relative pronoun's job is to hook a clause onto a noun that came before it, and the choice of pronoun depends entirely on what kind of noun that is. People take 'who' ('the engineers who designed the span'). Things, animals, and groups take 'which' or 'that' ('the bridge that collapsed'). Mixing these up, as in 'the engineers which designed the span,' is wrong for the same reason 'she don't' is wrong: the word doesn't match what it's attached to.

Inside its own clause, a relative pronoun plays a role: subject or object. 'The book that I borrowed was overdue' has 'that' standing in for the object of 'borrowed' (I borrowed the book). When a relative pronoun is the object of its own clause, it can vanish entirely: 'The book I borrowed was overdue' is just as correct with nothing there. But when the relative pronoun is the subject of its clause, it can't disappear, because then the clause would have no subject at all. 'Where' only works for literal places, not for abstract nouns like 'point' or 'situation'; those need a preposition plus 'which,' as in 'a point at which' or 'a situation in which.'

This topic sits next to modifier placement and clause joining because relative clauses are themselves a tool for attaching information without starting a new sentence. Getting the pronoun wrong often signals the same misreading that causes dangling modifiers: losing track of which noun a phrase is actually describing.