Needn't/Don't have to/Don't need to + present infinitive: No es necesario hacer algo (en el presente o en el futuro).
You don't need to take the dog for a walk, I will do it.
Didn't need/Didn't have to + present infinitive: No era necesario hacer algo. No sabemos si fue hecho o no.
You didn't need to/have to pay the whole amount today. (We don't Know if she paid it or not).
Needn't + bare perfect infinitive: No era necesario hacer algo pero se hizo.
You needn't have typed the whole thing again it was saved on the computer. (You did type it all).
