Informal it's for your friend/family member/ young people or old people when say that U can use informal

Formal it's for people that U don't know/ old or formal situations like school or work

I hope that you understand :)

the formal form is used when you talk to old people that you don't know, and to people who needs "respect" for some reason, also if you know them (f.e. older people, people that you don't know so well, important people or ones with who you're not so confidant)

practice it, it' s very important and very used :) please correct my english if you see any mistake

https://it.wikipedia.org/wiki/Allocuzione

Questo può chiarirti le idee.