su / sus

Question

"Mi padre no es ________ maestro". The question is "my father is not THEIR teacher" - so why is the answer "su", why not "sus" when it is their not his/her?

Accepted Answer

Hola Giles
this is because "su" means "his/her/your formal/their". It can be any of these.
These possesive adjectives agree with what comes after, i.e. the possesion. So if we are talking about "their teacher", teacher being singular, you need "su".
To use "sus", the sentence should say something like "those men are not their teachers":
Esos hombres no son sus maestros. 
I hope this clarified it
Inma

Answer

Because maestro is singular.  You would use sus maestros if there were more than one teacher.

Answer

su