Basic Changes of State, and Gases Question

[rubrboots]

Hello

I am currently working on a review before starting a senoir highschool chemistry course. I am now reviewing how to balance equations, which I do understand. But I dont understand how to determine the resulting state of the compound.
For example:

S8(s) + O2(g) ----------> SO2(g)

How do you know that the S (a solid) when combined with 02 (a gas) becomes a gas???

OR

Hg(l) + O2(g) ---------------> HgO(s)

liquid and a gas becomes a solid?

I am also wondering why oxygen is O , but the gas is 02. Same with Ammonium (NH4), but the gas state is (NH3).

I would really appreciate a quick explanation of these concepts, or the names of theories/subjects I should be investigating. Thank you very much.