I’m still trying to figure the middle out. It’s one of the hardest thing about the language, imo.
The websites are a bit unreliable, unfortunately. They often say “mp” due to the endings, whether the word is ever used as a middle or not. Inside the entries, it’s a different story. In an LSJ dictionary entry, for example, where it says “Med.”, you can generally rely on that, or in the Cambridge Lexicon “Mid.” and so on. These aren’t in the forms section of the entries, but in the meanings section. Ie. “δανείζω…2 || MID. (of persons) borrow” means that active δανείζω is “I do a δάνειον [loan]” (I lend), but middle δανείζομαι is “I do a δάνειον for me” (I borrow).
Now, since the dictionaries have mostly chopped up every text that you’re ever going to read, you can get by on middle forms by getting to a word in reading, looking it up in the dictionary, and checking what the dictionary says. This is what most 99% of people do, in fact.
The explanations in the grammars have a combination of useful and less useful information about the middle. I like Goodwin’s explanation from 1242 forwards as much as anything, especially at an early level, and he needs to be read. Link: https://archive.org/details/greekgrammar00gooduoft/page/266/mode/2up
If you want to understand the middle like an Attic Greek understood it, a living and generative element of his language which he could use without resorting to a dictionary even once, and coin new appropriate uses of on the fly…well, you have to read lots and lots. Maybe that will be enough, I don’t know. Like I said, I’m still getting there. The spirit of the middle, I begin to suspect, is related to a different consciousness of man in his world that the ancients had from what we do. While an active verb discusses a gross action, a middle verb is about more than simple self-reference, but is about conceiving the agent as an en-souled creature whose intention and experience are taken into account by the action. Εχ.
ἀποπέμπει - He sends off
ἀποπέμπεται - He sends away
Mechanically, this is the same action. But they have different contexts. The first can be used anywhere the second is used, but the second only where there is a mental image of the agent and his presence, which he is clearing of whomever is sent off. If he says “get out of my sight”, he is doing the second. If he says “I need you to be in Astoria tomorrow to work on a contract”, he is doing the first.