To the best of my understanding (as a relatively new Korean learner) nouns without plurality particles can be taken as singular, plural, or as the general concept of that noun depending on context. So, 남자 can actually refer to a man, a group of men, or to the general concept of a man or men. For instance, "남자 돼지야" could mean "Men are pigs" in general or "The man is a pig". 들 is used when you want to make a noun explicitly plural, likely to remove some sort of ambiguity or apply more context.
you need context without the "deul" to know if its plural or non plural. "nam ja" can mean both man or men. It depends on the context like I said before. This language study is under beta, so it's not going to be perfect. So Duolingo is great as a stepping stone into the language, but you are going to need other sources to help cement your knowledge and fill in the gaps that duolingo provides
또=tto pronounce the tt as if you are tutting and o very posh 는=neun pronounce the eu as oo
This is wrong 또는 should be pronounced as tto-neun (the eu makes a oo sound)