In English, we practically never use "thou" or "thee." Heck, not even "ye" - and it costed us our ability for a plural "you". Nevertheless, worrying about formality isn't a huge deal in English, since normal speech is already formal, although we do use words like "sir" and "miss". However, I know that other languages have varying degrees of formality, like "usted" being used with practically everyone except family and children one one end of the spectrum, and "u" being almost insulting if you call some Dutch people that.
Anyways - what's the difference between formal speech and casual chit-chat in the language you speak natively or study?
s'il te plait I'm talking comme je think. Tu peux me fight encore de what I think mais ses just me about it mais I'm Acadien atm un peut. As tu ever talked to des personnes bilangue au max?