Na and san mean pretty much the same... na is litterally "[it] is" but its only rarely used... san means "it" and is usually used for "it is" in Sindarin.
Mae (or lenited Vae, I dont think it should be mutated here but it sounds nicer) means "good" or "nice" or is just a positive word... like "great" it, while "Daer" and "Beleg" (and "Tond"😉 are the litteral words for "great" but rather in the sense of "big".