- Why is the name pronounced ثامر like with a ث?
- Why is there a ُ between تامر and أمريكي?
- I know that adjectives take certain suffixes, but why does أمريكي take an ون and عربي does not?
Regarding the first one, I think it's an error, so I'd report it.
For the other two, Arabic uses three case suffixes, -u, -a, -i, which can also be in indefinite form, -un, -an, -in. So far in the course, it seems like different case endings are appearing randomly :/
2amirikiyy-un is actually correct here, and endings are optional on the last word before a pause in speech, so that's why 3arabii isn't 3arabiyyun, which it would normally be.
To make things more complicated, most names are indefinite, so come with an automatic -un ending, but a few are known as "diptote", and don't use the -n, just -u. I'm not sure, but Taamir might be one of those names, hence why it's Taamiru here. Or it could be an error.
Thank you for the explanations! Good to know the rule that endings are optional on the last word. I’ve continued the lesson, so I guess I can’t report it anymore. I find this a serious issue with Duolingo actually...