"The cheese is for lunch."

Translation: O queijo é para o almoço.

[Klyamada]

Any tips on when to use "por" or "para " when translating the word "for" in English?

[Paulenrique]

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

[SmarandaLu]

Why is para o almoço correct, but not pelo almoço?

[ronkisimo]

same question?

[Paulenrique]

In this case, "pelo" = in behalf of

[Rodrigo_Duo]

Why the exercise don't uses article "the" : "The cheese is for the lunch"?

[Paulenrique]

English language does not require definite articles as Portugues does. In this case, you can omit "the" before "lunch".