What are the differences between colloquial English and English in common use?
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.
If something is in common use, it could probably be used in formal contexts, while something colloquial would not.
Your understanding of "common use" is wrong.
If it's in common use, it's used commonly.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.