It's the nominative indefinite ending, -un, which is rarely written, but if it were, it would be written as ٌ
Notice how the small symbol ٌ looks like two small وو linked together. The grammar behind when we use what when talking about indefinite things is a bit too involved to explain here, so what I suggest you do is pay really close attention to what you hear while following the Arabic letters with your eyes, and you'll eventually get a feel of what to use when. It takes a long time to acquire this skill, so just be patient and listen to a lot of Arabic while following text every day. You'll get there eventually :-)
PS: We also often drop the -un sound when reading out loud, so don't worry too much about it.
Ah, great! Yes, I had heard about the -un ending, but in this case, every ending sounds like -tun instead of -un, and I was wondering if that was the same or not. OK, I'll try not to worry too much about it, but at the start, as I want to understand what the audio is saying, all the -TUNs (or -UNs) are confusing me, haha... I thought the -UN was pronounced in the sustantive, but I see that the sustantive and all its adjectives have -UN (excepting the last one). Thanks for the tip. I'm enjoying this arabic course. It's a shame that the course is relatively short. Do you know if the course will grow at any point, or is it definitively finished? شُكْراً
The T sound comes from the letter ة which isn't pronounced when there is no ٌ , so once ٌ is added, its sound pops up again :-)
As for the course, I'm not a contributor to this course, so I don't have a definite answer, but judging from the many years I have been on Duolingo, I can say with reasonable certainty that they'll keep updating and growing the course with time. Arabic is a challenging language to teach though, both pedagogically and technically in terms of programming (the computer). So I would expect such growth to take more time than it does other courses.
Meanwhile, I suggest you listen to a lot of Arabic every day, even if you don't understand any of it, to help you progress faster/better. If you'd like general tips on learning languages, I recommend you read my previous posts on the subject, which you can find by following these links:
Thanks a lot, SamirShaker! I really appreciate your help and tips. I wish the Arab course will grow soon (I've done about 50% of it already!). And thanks for the links. Very interesting stuff. Man... More than 2000 days on fire. Wow! I've never reached a month in Duolingo, haha.