Preposition placement in relative clauses

There are often prepositions in relative clauses, and a relative pronoun is the object of that preposition. In everyday English, the preposition is normally placed at the end of the relative clause and the pronoun may be included or omitted. In formal English, the preposition is placed before the relative pronoun, and in this case the pronoun cannot be omitted. In the examples below, the pronouns in parentheses can be omitted.

Examples
Everyday English Formal English
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.