You can use the relative pronouns "who" and "whoever" to refer to the subject of a clause or sentence, and "whom" and "whomever" to refer to the objects of a verb, a verbal or a preposition.

In each of the following sentences, the highlighted word is a relative pronoun:

You may invite whomever you like to the party.

The relative pronoun "whomever" is the direct object of the compound verb "may invite."

The candidate who wins the greatest popular vote is not always elected.

In this sentence, the relative pronoun is the subject of the verb "wins" and introduces the subordinate clause "who wins the greatest popular vote." This subordinate clause acts as an adjective modifying "candidate."

In a time of crisis, the manager asks the workers whom she believes to be the most efficient to arrive an hour earlier than usual.

In this sentence "whom" is the direct object of the verb "believes" and introduces the subordinate clause "whom she believes to be the most efficient". This subordinate clause modifies the noun "workers."

Whoever broke the window will have to replace it.

Here "whoever" functions as the subject of the verb "broke."

The crate which was left in the corridor has now been moved into the storage closet.

In this example "which" acts as the subject of the compound verb "was left" and introduces the subordinate clause "which was left in the corridor." The subordinate clause acts as an adjective modifying the noun "crate."

I will read whichever manuscript arrives first.

Here "whichever" modifies the noun "manuscript" and introduces the subordinate clause "whichever manuscript arrives first." The subordinate clause functions as the direct object of the compound verb "will read."