Absence of necessity
Needn’t/Don’t have to/Don’t need to + present
infinitive: It isn’t necessary to do sth (in the present/future). You don’t need to take the dog for a walk, I will do it.
Didn’t need to/Didn’t have to: It wasn’t necessary to do sth. We don’t know if it was done or not. She didn’t neet to/have to pay the whole amount today. (We don’t know if she paid it or not.)
Needn’t + bare perfect infinitive: It was not necessary to do sth but it was done. You needn’t have typed the whole thing again it was saved on the computer. (You did type it all.)