مشکلی نیست

It is neutral. You can use it with your boss, a waiter, or your best friend. It's the safest choice for learners.

You will be understood, but it sounds grammatically incomplete. The '-i' (Moshkeli) makes it sound natural.

'Eybi nadare' is slightly more informal and often used when something is physically wrong or a mistake was made.

Change 'nist' to 'nabud': 'Moshkeli nabud.'

Yes, but only if the person is thanking you for something that might have been a hassle. Otherwise, use 'Khahesh mikonam.'

Yes, it's fine for semi-formal emails. For very formal ones, use 'Mas'ale-i nist.'

That's the informal version. 'Nadare' means 'it doesn't have.' It's very common in spoken Persian.

No, it's much more polite than 'I don't care.' It means 'I am not bothered.'

If you say it sarcastically, yes, but it's usually a positive phrase.

Usually 'Mamnun' (Thank you) or 'Lotf darid' (You are kind).

No, it's exactly like the English 'sh' in 'sheep.'

Yes! In Dari (Afghanistan), it's very common. In Tajik, they might use 'Mushkile nest.'

No, it's standard Persian.

Just use a rising intonation: 'Moshkeli nist?'