Ask the wise old owl
You mustn’t or you don’t need to?
You mustn’t = you are not allowed to do something. You should not do it.
You must not turn up without an appointment. Don’t turn up without an appointment.
You must not be late for this important meeting. Don’t be late.
You must not pick the flowers. It is forbidden!
You don’t need to = it is not necessary.
You don’t need to go to the shops today. I’ve already been. I’m not saying you shouldn’t go to the shop, but it isn’t necessary because I’ve already bought what we need.
You don’t need to feed the dog. I’ve already done it.
I sometimes have to correct sentences in which people have used “you mustn’t”, where they really mean “you don’t need to.
You don’t need to pick me up from the station. I’m happy to walk.
You can’t use “you mustn’t” here. It’s not forbidden for you to pick me up from the station, it’s just that you don’t have to do it because I’m happy to walk.