"We don't want any new walls!"
Translation:Wir wollen keine neuen Mauern!
In this sentence, it might make you sound as if you were saying Wir wollen keine neue Mauer (singular instead of plural).
But in general, if you use the wrong gender/number/case ending on an adjective, it will typically still be understandable but, of course, sound definitely wrong and mark you as a foreigner/learner.
A bit as if you said "I is giving him a apple" -- you know what they mean but you'll immediately notice the mistakes.
Can native speakers actually hear all of these endings? Do "keine neuen" and "keinen neuen" sound different enough to notice?
I've watched the pub scene from Inglorious Basterds countless times trying to pick apart the German being spoken, and one line always stands out - "Und ab die Entfernung, bin ich ein richtiger Frederick Zoller." I think that's what he's saying anyway. Without coming to a dead stop, the gap between n and r will include an extra noise, so it sounds like "eine richtiger," and the -r ending vanishes in conversation because that's what r-sounds do without a vowel to follow them, so for all I know he actually is saying "eine richtige" to, what, take a dig at Frederick Zoller's masculinity for some reason? Maybe this is a side effect of the laziness of American English, but seriously, can native speakers distinguish between "ein richtiger" and "eine richtige" well enough that you notice when it's wrong?
Obviously it doesn't change what's correct and incorrect, but I am curious if I really need to learn all four dozen or however many ways to say the same word (ja, ich bin salzig) or if there's an acceptable level of winging it that Duo isn't going to tell us if all I want is to be able to speak and understand conversational German.