How would you write the formulae for things like,
1. potassium sulfate
2. magnesium sulfate,
i'm really stuck and my teacher expects me to understand it.

Potassium Sulfate = K₂SO4

SO4 = negative 2, so you need 2 K+ ions to counteract it, hence K₂

There is a simple crisscross method for the same ->
Element-Potassium(K)Sulfate(SO4)
Valency - +1 -2

Now, write the valencies of a elements in the subscript of other..Means write the valency of sulfate in subscript of Pottasium and valency of Potassium in subscript of Sulfate. Like this - K₂SO4..Similarly Magnesium sulfate is MgSO4