Nouns name things.

Singular & Plural

When there's only one of something, it's singular.
spaceship, MacBook

When there's more than one, it's plural.
spaceships, MacBooks

The Article

The article introduces the noun.
a spaceship, the MacBooks

Pronouns

Pronouns are words used to replace nouns.
he flew the spaceship, she typed it for me

Relative Pronouns

Relative pronouns relate the person or object to the action.

Where the relative pronoun identifies a person, use who/whom or that.
Tim is the man who fixed my fence.

Where the relative pronoun identifies an object, use which or that.
This is the book that I read.

Relative pronouns can often be left out completely.
This is the book [that] I read.

Where the relative pronoun tells you more about an already identified person, don't use that and don't leave the pronoun out.

Nouns 1 | Nouns 2