"These letters are from you."
Translation:Surat-surat ini dari kamu.
Maybe think of "ada" like "estar" in spanish if you know what I mean. It's not a copula that links a noun with another noun or adjective like "ser" - it's an intransitive verb that means "be" in the sense of "exist". You could say "suratsurat ini ada" by itself and it would mean "these books exist".
Kalian is a plural second person pronoun like you(plural), ye, or y'all. Kamu is a singular second person pronoun like you(singular) or thou.
Both kita and kami mean we.
Kita is an inclusive first-person plural pronoun. It includes you and the person or people to whom you speak.
Kami is an exclusive first-person plural pronoun. It excludes the person or people to whom you speak.