Sometimes we use a pronoun instead of a noun, if the meaning is clear.
The English pronouns include words like 'I', 'you', 'them', 'us', 'mine', 'his', 'this', 'those', 'one', 'ones', 'someone', 'anything', 'everywhere' and 'nobody'.
Here's a list of all the explanations and exercises about pronouns.
Subject and object pronouns explanation
Subject and object pronouns exercise
Possessive adjectives and possessive pronouns explanation
Possessive adjectives and possessive pronouns exercise
Indefinite pronouns explanation (something, anyone, nowhere, everybody, etc.)
Indefinite pronouns exercise (something, anyone, nowhere, everybody, etc.)