1. (adj) su with singular noun

2. (adj) sus - that's their car: ése es su coche one of their cousins: un primo suyo uno de sus primos with plural noun

3. (adj) - tell them not to put their heads out of the window: diles que no asomen la cabeza por la ventana they've left their books behind: se han dejado los libros everyone must do their best: cada uno debe hacer lo que pueda with parts of the body, etc.