When to use "everyone has" and "everyone have" please.
When 'everyone' is the subject of a sentence, by convention, ['convention' means that is what the vast majority of English users use] it use a singular verb, eg. has/is/goes/eats/is heading/ etc.
When 'everyone' is used as a "name", it can be followed by 'have'.
Children, have fun.
Bill, Jill and Phil, have fun.
Everyone, have fun.