Pronoun

A pronoun is a word which replaces a noun.
Can you see my phone? I need it.
Sam is here. Yes, there she is.

The most common pronoun type is the personal pronoun. For most of these, the subject form is different from the object form.
* I, me
* You, you
* She, her
* He, him
* It, it
* We, us
* You, you
* They, them

Each of these also has a [possessive] form, but that form is used as an [adjective] rather than as a noun.
Have you seen my glasses?
I do not have your book.

There is a separate form of the possessive personal pronoun. Not that none of these has an [apostrophe]:
* mine
* yours
* hers
* his
* its
* ours
* yours
* theirs
We use these when we want to use a possessive without a separate [noun].
Those are not my glasses: they are yours.

Another common type of pronoun is the relative pronoun. We use this type in [adjective clauses] and [adverb clauses].
* which
* who, whom ([object])
* that

In [noun clauses], the group is slightly different:
* what
* which
* who, whom ([object])

English has another personal pronoun to talk about a non-specific, often imaginary person.
If one wishes, one may leave now.
This form is rarely used, and tends to sound very formal. Its form is unusual because its [possessive] has an [apostrophe]:
* one’s