Any tips on when to use "por" or "para " when translating the word "for" in English?
Preposition varies a lot from language to language so it's hard to give a bunch of rules on it. If something is "toward/directioned to" translate "for" as para and when it is "in behalf of someone/something, by, per, a (day)" translate "for" as por
Why is para o almoço correct, but not pelo almoço?
In this case, "pelo" = in behalf of
Why the exercise don't uses article "the" : "The cheese is for the lunch"?
English language does not require definite articles as Portugues does. In this case, you can omit "the" before "lunch".