I think many people are too focused on getting marked right than learning how Italians use their language. WHO CARES IF YOU'RE MARKED WRONG? Nothing bad happens to you, this is a game/an app, you won't get more or less money, you won't lose anything tangible, relax. We're taught in language learning that mistakes are ok, even good sometimes. Take risks and learn from the mistakes.
I think you'll be fine if you use "stare" when you would use "stay" in english, e.g. if you stay at certain place or for a period "I stay at school even after classes." "I'll stay for the weekend", and "rimanere" - "remain" if a certain state has been kept, or after something is left behind or you still have something e.g. "I will remain ill if the drugs don't work." "My brother remained sick at home.", "I remained with only 20 Lingots."
Yes, "stare" is irregular, and both forms are used.
doesn't this break the rule that, quoting DL, "for affirmative commands fo 'tu' you use the 3rd person singular in the present for verbs with -ARE". Therefore, if the rule is valid, using stare this should be 'sta qui con noi' shouldn't it? Or am I missing something?