Don't pay too much attention to the diacritics.
I see plenty of posts about why it ends with -un and not -an, or simply -n.
Let me put your minds at ease. Unless you're aiming to be a poet or writer. You can completely ditch these nuances. I know it can be daunting, especially for a beginner. But believe me as I tell you that if you say "Orid an atakallam ma3ak" (I want to speak with you) instead of the correct form "Orid-u an atakallam-a ma3ak-a (or k-i for female)" أريد أن أتكلم معك It makes little ignorable difference. And your interlocutor will definitely understand the message. And what's language used for but to convey a message !!
So, don't sweat it too much if you can't get your head over it. What you should master as a beginner are the differences of writing of alphabets in terms positions as it would be egregious to write أري د أ ن أت ك ل م م ع ك for the same sentence above, Pronunciation and finally expanding your vocabulary. That's it. Pay attention to these for now. As a casual learner, that's all you need, and if you're doing this to pass some test or something, then you better look up other resources as well.
Just wanted to say thanks for mentioning this. I've wanted to say the same when I saw -un, -an and -n being added every single time, but I'm no language expert and wasn't sure how to explain it. I think it makes it harder for beginners to focus on small things like that (which I think is part of Classical Arabic[aka Fushah] instead of MSA), when they should focus on connecting letters, pronunciation and vocabulary.
It would be helpful if they added that somewhere in the hints or managed to prevent the audio from pronouncing it altogether.
Personally, I do intend to figure out how to use it sooner or later (though I'm currently still in the "learning to read" phase), but I can see how lots of people are going to be confused if the audio says something that they can't actually see written on the page.
I've noticed that the text to speach doesn't seem to use these suffixes correctly as sometimes accusitive endings (-a,-ta etc) are used insted of the nomitive ones (-u,-un etc.).
This could cause more confussion among learners as this feature of the languge isn't explained in the skill summary. By luck I had read about if beforehand in a text book but hearing the audio in duolingo could be very confusing for anyone.
Thank you for pointing out that this isn't an important feature for common use of MSA.