I'm going to make an attempt at explaining this. “Multa” ends in -a so it's an adjective. That means it'll always be connected to a noun, e.g. “multaj homoj”, “multaj birdoj” etc. “Multe”, on the other hand, is an adverb, since it ends in -e. That means it'll always complement a verb, e.g. “Mi ne havas multe da bieroj.”
I'm sorry, I still don't get it fully. Do you mean to say that "multaj x" will always be a subject, whereas "multe da x" will always be assumed an object (i.e. always relate directly to the verb)? (Here's the grouping in my head: ((Mi havas)multe da bieroj) (making "multe da x" a weird noun-y phrase I don't quite understand that's also meant to be an adverb somehow) not ((Mi havas multe) da bieroj) (which looks incredibly strange to me). Sorry, I come from yonder "toki-ponujo" (toki pona land), where rules surrounding verbs tend to get a lot stricter, e.g. there's debate around why it shouldn't be (EO rough equivalent of) "mi fartas bone" but "mi bonas farte". So, "havas multe" would translate to "have many-ly" rather than "have many" from that perspective. Then again, Esperanto uses adverbs in so many strange and new ways to me (time I can understand). Thanks for your help, though, and if it is like how I described earlier, please inform me.
Hi AetherStar! Like I said, I am no master at Esperanto, and I am confused by many things like that too. Learning a language takes a lot of input and some curiosity too, though.
(Oh, by the way, I don't speak toki pona but I know of it. Cool language!)
I was writing a very long answer to help you, but now I'm more confused than before! :S Sorry, but I can't help you at the moment. Now I'm mixing up multa, multe, multe da and tre! I hope a person who knows more than me comes and saves us both :P
Hi! So sorry about that!! :( I posted the question on Reddit too. Someone there told me that "multaj x" and "multe da x" are interchangeable, it's just your preference. And I'd personally say that "multe" on it's own is just "a lot" (e.g. Mi laboras multe = I work a lot, Mi mangxis multe = I ate a lot, v.s. Mi mangxis multe da pomoj = I ate a lot of apples...) That's all just what I've heard plus my own thoughts though. Let's hope that's just it, and if there's something else needing to be set straight, I'm sure someone else will follow up. Sorry for all that!! :/