I wrote it is a plate, and it is correct. Is the third person masculine singular then both used as it and he?
That's a very big question! And I'm sure that duolingo has notes on it somewhere (I'm very bad at finding duolingo's notes) -- but here's one introduction: http://www.nualeargais.ie/gnag/kopul2.htm Ignore the fact that the author can't spell "existence": it's quite a good explanation if you're of a grammatical bent. Note that the distinction is not at all unlike that between "es" and "está" in Spanish, if you know anything of that language. I see you're also learning Welsh: the same distinction is observed there too between saying WHAT something is -- Beth YW e? (what is it?) -- and HOW/WHERE etc. it is -- Ble MAE e? (where is it?).