The value of the "uma" and "um" here is in emphasizing the feminine and masculine nouns and retaining this in memory. Using the "a" before pencil here is optional but since it is in the Portuguese one might just humor the situation and use it in the English. While it is optional in English, as pointed out by Jayeless, it does distinguish between two separate meanings which the out of context sentence does not inform us otherwise.
That started off good...then went off the rails a little.
Watch: umA caneta........um lapis.
See what we're being taught?
"You need the article "a" here." No Duolingo, as a native English speaker I can safely say I don't :/
I think that sometimes they just want the literal translation. The "uma" and "um" are not strictly necessary in Portuguese either, but they were put there. =)
Why not? It's not because it's your mother tongue that it means that you are an expert in that language.
Well, unless you're a linguistic prescriptivist, it kind of does...? In linguistics (at least in Australia, where I've studied it) the emphasis is always on what seems natural or unnatural to native speakers, not on artificial standards.
As for why you don't need the second "a" here - to me either sentence is grammatical, but mean different things. "Do you have a pen or a pencil?" is like, "I know you have one, now tell me which." Whereas "Do you have a pen or pencil?" means you don't know if they have either, but you'd like to borrow something to write with. The second sentence is more useful so that's why I put in that one. It's certainly wrong to say you NEED the second "a" there, because I'm sure I've heard this sentence (in English) dozens more times without it than with. (In fact, I'm not sure I've ever heard it with.) So, Duolingo should fix that :P
Article added seems fine to me. In fact, it's grammatically better. Some people would find natural to say, 'He don't know nothing.' Yet that's wrong in like 100 levels.
Yet in a hundred years it may be found in written language as well if that use spreads. Language is in flux. Unless it's called Icelandic, that is.
You are right. This sentence could be both
Do you have a pen or a pencil? (You already know they have either a pen or a pencil, but you do not know which)
Do you have a pen or pencil? (Unsure of whether they have both or just one, so you asre asking which one they have)