"You do not live in the mountains, Omar."
Translation:لا تَسْكُن في ٱلْجَبَل يا عُمَر.
-- I have the impression from literary readings and journalism that the singular form "jabal" can often mean something like "the mountainous terrain around here" or "the mountainous terrain in the context of the dialogue the speaker is involved in" -- the Hans Wehr Arabic-English dictionary gives as translations "mountain; mountains, mountain range", and cites as plural forms جبال jiba:l and اجبال ajba:l -- the first of those (cited adjacently by TJ_Q8) is probably much more common -- maybe someone can post some citations to support my impression, if it is indeed correct -- Duolingo certainly seems to think الجبل can mean "the mountains" -- I've encountered something analogous in some Spanish-speaking regions (e.g. in at least some parts of Mexico and Guatemala) where the singular form "en el monte" often means "in the nearby mountainous terrain", which actually has multiple peaks or is a range -- Spanish also has the corresponding plural form "los montes", which might be used when it's desired to focus on multiple separate peaks, rather than on a stretch of mountainous terrain --
Yes, it can be so. Using singular form for the general meaning.
The thing that most people often forget is that you can't translate completely word-to-word between any two languages. Simply, the logic that each culture bear in mind while using the language is different.
In previous posts around this forum I've pointed out how in English you would sometimes drop the "THE" article when talking about general aspects, but still in Arabic no such thing happens; e.g. I love Nature which translates to Arabic أحب الطبيعة (uHibbu aT-Tabí3ah). The word الطبيعة (the nature) is defined while it's not so in English. The mechanics for each language is different so when learning should have an open mind about such differences in logic in usage