I'm trying to rewrite the sentence below to indicate ownership by using possessive adjectives. I caught the first error with the verb ('son'), but I don't understand the adjective error. In my mind it translates as: "It's their ('sus') tape recorder," so why is the singular ('su') adjective correct? Am I confusing something in agreement?
Sentence to rewrite: "Ellos compran una grabadora." My Error: "Son sus grabadora." Correction: "Es su grabadora."