Learn Classical Arabic Vowels الْعرَبية
Hey, I'm learning Classical Arabic and made a deck for those wanting to know the Vowels. They can be quite difficult and Arabic and be hard to understand without them!
So the tanwin is used when things are indefinite. As far as the 'un' part of your question, the 'un' sound indicates the indefinite nominative case. All the cases are very common. I'd guess you're seeing the 'un' more frequently because you're looking at beginner materials like flashcards which have words out of context.