WHO replaces nouns and pronouns that refer to people. It cannot replace nouns and pronouns that refer to animals or things. It can be the subject of a verb. In informal writing (but not in academic writing), it can be used as the object of a verb.

WHOM replaces nouns and pronouns that refer to people. It cannot replace nouns and pronouns that refer to animals or things. It can be the object of a verb or preposition. It cannot be the subject of a verb.

WHICH replaces nouns and pronouns that refer to animals or things. It cannot replace nouns and pronouns that refer to people. It can be the subject of a verb. It can also be the object of a verb or preposition.

THAT replaces nouns and pronouns that refer to people, animals or things. It can be the subject of a verb. It can also be the object of a verb or preposition (but that cannot follow a preposition; whom, which, and whose are the only relative pronouns that can follow a preposition).

2. The following words can also be used as relative pronouns: whose, when, where.

WHOSE replaces possessive forms of nouns and pronouns (see WF11 and pro in Correction Symbols Two). It can refer to people, animals or things. It can be part of a subject or part of an object of a verb or preposition, but it cannot be a complete subject or object. Whose cannot be omitted. Here are examples with whose:

The man is happy. + I found the man’s wallet. =

The man whose wallet I found is happy.

The girl is excited. + Her mother won the lottery. =

The girl whose mother won the lottery is excited.

WHEN replaces a time (in + year, in + month, on + day,...). It cannot be a subject. It can be omitted. Here is an example with when:

I will never forget the day. + I graduated on that day.=

I will never forget the day when I graduated.

The same meaning can be expressed in other ways:

I will never forget the day on which I graduated.

I will never forget the day that I graduated.

I will never forget the day I graduated.

WHERE replaces a place (in + country, in + city, at + school,...). It cannot be a subject. It can be omitted but a preposition (at, in, to) usually must be added. Here is an example with where:

The building is new. + He works in the building. =

The building where he works is new.

The same meaning can be expressed in other ways:

The building in which he works is new.

The building which he works in is new.

The building that he works in is new.

The building he works in is new.