Actually it is supposed to be written but knowing Duolingo so far, there are lot of mistakes (specially in the text-to-speech machine). This -un you hear is actually a vowel on its own and given by the sign ـٌ or sometimes as ـُُ. It is called Tanwin, or Nunation in English. Older extinct languages like Akkadian, had something similar but called Mimation (-um). This vowel plays great part in many things but anyway for the time being you can consider it as a sign for indefinite noun (i.e. it is somehow similar to a/an in English). For this reason, you cannot put the (AL) -the definite article in Arabic- with Tanwin in a single word; They don't fit together.
No not that. Just Tanwin does not come with the definite article (AL) in a single word. For example:
- A book: كِتابٌ (kitábun).
- The Book: الكِتابُ (al-kitábu).
It can never be الكِتابٌ (al-kitábun). Nunation cannot be put with the definite article AL in a single word.
And just an extra note, the final vowel in words can be dropped when the words come at the end of the sentence and the speech ends. So, we can simply say الكتاب (al-kitáb) or كتاب (kitáb) without saying the last vowels (-u) or (-un) respectively.
to what I see, in general, it seems that this is a general font design problem with the Arabic script specifically. The only solution might be to change the font to one which is bold or something ... anyway, i don't know much about android and iOS but on PC or laptop one can always enlarge the text by pressing CTRL and + while on the page. Also, in the browser, there are options and settings to change the font of the display and its size and even force the browser to use specific fonts instead of the default font of the page.
This is Arabic. There are symbols called 7arakat دَجَاجَةٌ dajajatun When you practice reading, you can do without it. Unfortunately, I did not like the way to learn Arabic is not sound. Also, there are meaningless terms that are repeated many times that do not exist in Arabic if you use correct words At least the learner benefits from the words on the occasion.