Question

Informal and formal words, is there any rule in using them? Like we can't use informal words in letter or wat...?

Answer 1

There are guidelines, rather than "rules." Basically, you should always consider the writing context -- who you're writing for, with what purpose, and in what form -- when deciding what level of diction to use.

Answer 2

Always ask the teacher questions if you do not understand what the teacher is looking for. Informal words could mean: "Don't use slang" and "Don't use contractions."