I eat fruit for breakfast.
I translated this as "Yo como fruta para desayuno." It was incorrect and said the correct answer is “Yo como fruta por desayuno." Makes no sense to me because when I hover over the word “for” in English, one of the drop down words is para, not por. When do you use which (por vs para)?
You're still thinking in English when writing in Spanish. Each language has its own structure, so let it be, don't try to match the sentences word by word.
This is very bad Spanish>>> Yo como fruta para desayuno.
This is not the most natural translation >>>> Yo como fruta por desayuno.
to eat/have breakfast >>>> desayunar
to eat/have fruit for breakfast >>> desayunar fruta (in Spanish we don't need "for")
Best translation: I eat fruit for breakfast >>> Desayuno fruta
Your answer was less wrong than the right answer. It is "(Yo) como fruta para el desayuno" or, more freely, "Como fruta al/de desayuno". "Por" is impossible there. Please, report it.
(Yo) Como fruta en el desayuno, or as King A said, (Yo) Desayuno fruta. In the first sentence, "desayuno" is a noun; in the second, a verb.
"Para" might be used in phrases like Tengo fruta para el desayuno. Hope this helps :)
Using the word "para" the best translation that I can think of is: "Yo como fruta para desayunar". But "yo desayuno fruta" and "yo como fruta de desayuno" are probably the best translations.