The relative pronoun is usually omitted when it is the object of the relative clause.

Selfless service to humanity is the best thing (that) a man can do. (Here the relative pronoun that is optional.)

This is the girl (whom) I love most. (Whom is optional in this case.)

It is easy to find out whether a relative pronoun is the object or the subject of its clause. If it is the object it will be immediately followed by another noun.

Compare

The girl who won the first prize is my neighbor’s daughter. (Here the relative pronoun who is the subject of the relative clause ‘who won the first prize’.)

The girl who you are talking about is my neighbor’s daughter. OR The girl whom you are talking about is my neighbor’s daughter. (Here the relative pronoun who/whom is the object of the clause ‘who you are talking about’. It is followed by the subject – you.)

The antecedent (the noun the relative pronoun refers to) may be omitted in some sentences.

(He) who laughs last laughs best. (Here the antecedent he can be omitted.)

(Those) whom the gods love, die young. (Here the antecedent those can be omitted.)

The position of preposition in a relative clause

The preposition can be placed either before the relative pronoun or at the end of the relative clause. The former method is considered very formal.

The boy to whom I spoke is my friend. (Very formal)

The boy whom I spoke to is my friend. (Informal)

In an informal style, the same sentence can also be written as:

The boy who I spoke to is my friend. (Who instead of whom)

The boy that I spoke to is my friend. (That instead of who/whom)

The boy I spoke to is my friend. (Omission of relative pronoun)