There may be exceptions to this rule, but usually ל marks an indirect object of a verb and it's a part of the verb definition. If there is no verb, or the verb does not take an indirect object, and you want to express that something is "for" someone, you use בשביל.
In this lesson there were a lot of positive statements using הוא and היא. We're being taught to always write האוכל הוא בשבילם and never האוכל בשבילם. So why doesn't this sentence need a הוא? Is the rule different for negative statements?
To me, the intermittent use of הוא and זה as pseudo-verbs (meaning "is") is the most confusing thing about this course. DL is very strict about using or not using them, but I have yet to see a clear explanation of where they're needed, aside from distinguishing indefinite sentences (E.g., "A book is good" versus "A good book".)
You never need הוא / היא. The only exception is comparing nouns to nouns. E.g., אפרסק הוא פרי
(im also learning, so no promise that thats all correct)
There are other cases where without הוא the sentence sounds very strange. It's not easy for me to formulate a rule. In the sentence above, you can put or omit the הוא both in positive or in negative. In positive both with and without sound to me about as natural; in negative, putting הוא sounds just a bit less natural.
I have the same question. My understanding is that this a grammatical device called a 'copula', which is sometimes needed because Hebrew lacks a specific word for 'is' or 'are' so sometimes, something is needed to indicate that what that follows is the (grammatical) object of the sentence. I haven't been able to figure out if there are actual rules for using it or if Duo is being quite arbitrary about it. I'd love to know if there are rules, and so, that they are. It might be a matter of context or 'feel', in which case it isn't really be fair to mark an incorrect answer, unless an explanation can be placed somewhere in the tips and notes.
Go with the advice of native speakers such as YardenNB. I, too, have been trying to figure out some rules, and so far here is what I have come up with, but I'm not a native speaker: (1) In general, use the copula when it's a noun + noun situation, but omit it when there is only one noun. I think if you go with that rule to start with you can then adjust based on specific idioms. It appears that using a copula with a negative when there is only one noun sounds less natural (YardenNB). (2) זה and הוא can be used as copulas but the former causes the sentence to be less formal. (3) The presence of a copula can affect definiteness and indefiniteness, as in Arabic. In the sentence, התפוח והתפוז רטובים, "the apple and orange are wet," the lack of copula means only those fruit are wet (definite). To use the copula in that sentence would mean that all fruits of that sort are wet (indefinite).
I don't think I agree with your last observation. Yes, התפוח והתפוז הם רטובים can mean apples and oranges in general, but this is very formal and outdated. In modern Hebrew we'll say (and write) תפוח ותפוז הם רטובים (well, it's indefinite, duh). And here the copula is (kind of) needed to make it a complete sentence rather than just a noun phrase ("a wet apple and a wet orange").