এটা কোনো বিষয় না

It is neutral. You can use it with your boss, your teacher, or your friends. It's very safe.

Yes, but be careful. It means 'I don't care' in a 'it's not important' way, not necessarily in a rude way.

'Bishoy' is slightly more formal/standard, while 'byapar' is more colloquial. They are interchangeable in this phrase.

It's better to include 'kono'. 'Eta bishoy na' sounds a bit incomplete, like saying 'This not matter' instead of 'This is no matter.'

You can just smile and say 'Dhonnobad' (Thank you) or 'Apni khub-i bhalo' (You are very kind).

Yes, it is equally common and understood in both regions.

No. Avoid using it for tragedies, health issues, or major financial losses.

The most popular slang version currently is 'Para nai' (প্যারা নাই), which means 'No stress.'

Usually, yes. It can also mean 'subject' (like Math or History) or 'topic.'

Yes, in Bengali, the negation 'na' almost always comes at the end of the sentence.

Yes! It's a very common way to be humble when someone praises you.

Yes, if the interviewer apologizes for a small delay or a technical glitch, it shows you are composed.