need help with writing out chemical formulas!

i understand that, for instance, you need one lot of Na (valency +) to balance with Cl (1-) to get a net charge of zero, which is very simple.

But what the heck do i do when i have, for instance, been asked to write out Magnesium Sulphate?, or Ammonium Hydroxide? << these ones are tricky for me because the one of the ions has two letters in it.

Help me out please, thank you

Well, Mg is 2+
Sulphate, SO4 is 2-

Ammonium Hydroxide is NH4Cl
NH3 has no charge, so it gains a hydrogen from acid (or water), becoming NH4+, which bonds with Cl-

It's basic periodicity

thanks, i get the idea now. i thought SO4 meant that the valency for the ion was 4+, but it was actually -2