"This is not your problem."
Translation:Это не твоя проблема.
Because you're referring to a random thing, you don't know the actual problem. If you'd say "this problem is not yours", it'd be эта проблема не твоя, because you're stressing: THIS is not your problem (but something else might be).
You use Ваша If you are talking to someone you dont know or someone older than you. Its more formal and respectful. Just like 'You' has its formal and informal addressing. So; Вы (you -formal) Ты (you- informal)
Im learning too, buy I think it it because it says "this is" rather than "this".
Эта проблема - this problem Это проблема - this is a problem. This is (это) is invariable, while just the "this (этот, эта и это) chаnges according to gender and case.