I don't know if this will help but listen more closely to the double consonant (not just here, this pronunciation difference seems to be prevalent).
I know this isn't quite correct and maybe a little "arty" but "она" flows. "Анна" definitely has a pause/stress in it as it lingers on the first н.
When in doubt, I always recommend Forvo:
I am still really struggling with this, despite all the tips given. I replay again and again, and eventually settle on one - but it's invariably the wrong one. :(
By the law of averages, you'd expect to get it right half the time, but I don't think I'm even managing that.
I'm sorry that you're struggling with this. Going a bit outside the norm (and assuming, despite you being in Bristol and speaking outdated English instead of American English ;-), the stress is the same)
It's all about the stress on the syllable. Perhaps not the best description and I'm sure someone will correct me but maybe this will help:
Она = ОН-а - stress is like climate (CLI-mate) - but it's much more fluid. This flows.
Анна = он-НА - stress is like require (re-QUIRE), it's less fluid there's a definite pause at the hyphen. It doesn't flow, there's not a full stop at the "НН" but it's pretty close. Like you're cheating a stop sign.
Thank you, I'm sure I'll get it eventually. On the scale of difficulties in learning Russian, I'm sure it's the least of my worries, but I do wish they'd use a different girl's name instead of "Anna". It's frustrating when you did get a case right, and negotiate a couple of other tricky spellings, only to get dinged because you thought "she" was "Anna", or vice versa.
Анна in this course has stress on the first syllable, and она has stress on the last syllable.
Американка is a noun, right? It might be more clear to those learners who are thinking about the details if the preferred translation were noun to noun (She is an American) rather than noun to adjective (She is American), like in the setup for the "Он американец" sentence.