Relative pronouns connect two clauses together, replacing a noun mentioned in the first clause to avoid repetition. (e.g., The man who is standing there. The house that I bought.)

CHE (Who, Whom, Which, That)

Che is invariable (never changes) and is used when the relative pronoun acts as the Subject or Direct Object. It is NEVER used after a preposition.

• Il libro che è sul tavolo. (Subject)
• Il libro che leggo. (Direct Object)

CUI (Whom, Which)

Cui is invariable and is used ONLY after a preposition (a, di, da, in, con, su, per, tra, fra).

• La persona a cui ho parlato (The person to whom I spoke).
• La città in cui vivo (The city in which I live).
• Il motivo per cui sono qui (The reason for which I am here).