chemistry equations help a-level

i did double science at gcse, so i never really got to grips with knowing the products of reactions such as metal oxides with acids ?
Im in year 13 now
can anyone offer help on how to know what the exact products are of a reaction?
for example
SO3 + 2KOH -> K2SO4 + H2O
how do you know K2SO4 and water are formed? like, i cant ever figure out what the chemical formula is of the product :/ like K2SO4?
im sorry if its really obvious and im probably missing fundemental knowledge but i cant see any explanation in the books

I suspect you're doing AQA, in which case you should know that SO3 is an acidic gas. So, you've got an acid and a base -> salt + water. You might ask why K2SO4, but your answer comes in terms of oxidation numbers. You might think SO3 might be an oxidising agent, but KOH isn't going to be a reducing agent, so the S in SO3 and K2SO4 should have the same o.n.. Which they do.