Lesson 52: Being done for someone with やる あげる もらう くれる
As a normal verb, やる is highly contextual, and you might as well treat it like its meaning depends on the structure it's in. It's a "do" word, but it's more general than する, and it has different connotations.
As an auxiliary verb, やる expresses that the action was in some way undesirable to someone involved, like doing something at their expense.
ケーキを食べてやった
I ate the cake (at someone's expense)
怒鳴り付けてやった
I shouted at him (even though he wouldn't want it)
あげる is a "give" word. In the structure てあげる, it means that someone did the action "for the sake of" someone else. But there's a catch: When it's the core of the independent statement itself, and not hidden away inside a relative clause or modified in such a way, it never refers to an action done for the sake of the speaker (nor for the sake of someone the speaker is speaking for).
There's no good term for this, but a new one that might gain some traction is "private predicate". If the speaker (or someone they're speaking for) is the person receiving the good will of the action, and the action isn't embedded or relative, the speaker can't use てあげる. A native japanese speaker will refuse, on a grammatical level, to think that the speaker is the receiver.
This is a way of avoiding ambiguity when talking about giving and receiving with dropped subjects. But even with an explicit subject, you can't use the speaker as the receiver of てあげる. You need to use くれる instead.
もらう is similar to あげる, but means "getting" instead of "giving".
もらう almost acts like a passive. The receiver of the action is marked as the subject and not the indirect object. This is what it means when I say it refers to "getting" and not "giving". もらう is also the opposite of あげる in that the speaker cannot be the giver of the action.
The speaker will not be marked as the indirect object of a もらう or あげる statement that the speaker is making, even though the speaker has different roles in both kinds of statements.
あげる is sometimes used when the giver and receiver are both third parties. Then the speaker is implying that they empathize with whoever was giving.
もらう means the receiver is the speaker or someone they empathize with.
くれる is a "give" word, but it works like the opposite of あげる. The speaker (or their party) must be the one receiving the action, and cannot be marked as the giver. When the speaker is the giver, あげる has to be used.
When the giver or receiver is not stated explicitly, you need to understand the rules above in order to understand who's playing what role. That's the only reason I bothered to explain what's going on. If it's hard to understand, skip it, try to learn it through exposure, and maybe come back here later.
Some linguists consider this a special case of subject-verb agreement. If you come from a language with pervasive subject-verb agreement, that might be a more intuitive way to think about this for you. But I don't know how well the idea fits, so if you want to think about this like agreement, be careful, I don't know what to warn you about.