Il y a souvent des prépositions dans les propositions relatives, et le pronom relatif est le complément d'objet de cette préposition. En anglais courant, la préposition est normalement placé à la fin de la proposition relative et le pronom relatif est facultatif. En anglais formel (surtout à l'écrit), la préposition est souvent placée avant le pronom relatif, et dans ce cas le pronom ne peut être omis. Dans les exemples ci-dessous, les pronoms entre parenthèses sont facultatifs.

Exemples
Anglais courant Anglais formel
Is that the man (who) she arrived with? Is that the man with whom she arrived?
Does he know the girl (that) John is talking to? Does he know the girl to whom John is talking?
The person (who) he is negotiating with is the Chairman of a large company. The person with whom he is negotiating is the Chairman of a large company.
It is a club (which) many important people belong to. It is a club to which many important people belong.
He liked the people (that) he lived with. He liked the people with whom he lived.
The tree (that) they had their picnic under was the largest and oldest in the park. The tree under which they had their picnic was the largest and oldest in the park.
It was the river (that) the children preferred to swim in. It was the river in which the children preferred to swim.
The jungle (that) the tribe lived in was full of strange and unusual animals. The jungle in which the tribe lived was full of strange and unusual animals.