when something is necessary, you cannot choose not to do it, it is a duty, something that you must do.
- To be able to live we have to eat, we have to sleep and we have to breathe.
Don’t have to:
when something is not necessary, not a must
- You don’t have to remember all of your friends’ phone numbers anymore because your phone has a contact book for you. You can if you want to but it’s not necessary, it’s not a must.