You mustn’t wear a tie to work if you are an English teacher. It’s just not necessary.
You don’t have to wear a tie to work if you are an English teacher.
Look at this baby. So cute! He’s standing all by himself for 5 minutes.
He has been standing all by himself for 5 minutes.
Miss, what did you do yesterday at 5 when your brother was attacked?
Miss, what were you doing yesterday at 5 when your brother was attacked?