question

could anybody explain to me the difference between the following sentences :
1.I needn't have done it;
2.I didn't have to do it.

I think there can be a small difference between the two

If you say 'i needn't have done it' you will always be talking about something you already did, and i think it indicates regret. 

If you say 'i didn't have to do it' you could be talking about something you didn't do, because you didn't have to. 

At least this is my interpretation of those 2 sentences :) 

question

could anybody explain to me the difference between the following sentences :
1.I needn't have done it;
2.I didn't have to do it.

They both mean the same thing, but I will tell you that at least in the USA, the second one "I didn't have to do it" is what you will hear 99.9% of the time.