I just got the sentence, "Los huesos de él son largos." Is this equivalent to, "Sus huesos son largos."? Why or when would you use one form rather than the other?
I'm not fluent, but I think the difference comes down to context: "sus" could apply to ella, él, or usted. Specifying "de él" eliminates some of that confusion.
Superdaisy is right, the first one is to specify the gender. But if it's in a text, for example, and the subject has already been mentioned before, than the second one is preferable.
The two are pretty much equivalent.
So how to to know when to use which..
It's just like in English. For example: Little Red Riding Hood went to her grandmother's house. Little Red Riding Hood picked some flowers on the way. Little Red Riding Hood got eaten by a wolf. Little Red Riding Hood was saved by a hunter.............................. So we don't say that. Because it's tiring and sounds like first grade writing. So pronouns were invented, and we put them in so to save us from saying to subject over and over again. That's how you choose which form to use. If the subject had already been introduced, then we can just use 'sus huesos son largos.'