Add extra information about a noun with who, which, that — and know when to drop them.
The information that tells us exactly which person or thing we mean.
Extra, non-essential information, set off by commas — My brother, who lives in Lviv, is visiting.
Where, when and why standing in for a preposition + which.
The book (that) I told you about — where the preposition goes, formally and informally.
Whose, whom, and relative clauses that comment on a whole previous clause.
Shortening a relative clause by dropping the pronoun and the verb be.