Reply to Alex's question

Question

I was wondering if there was a reply to his question below:"didn't need to (infinitive) & needn't have (past participle) are used to express the lack of necessity in the past, ..."

Accepted Answer

Hola Nicole
I am copying here what I replied to Alex (the answer was on a different thread):
For the first one "I didn't need to have an interview" we would say: 
Yo no necesitaba hacer la entrevista.
It is very straight forward in this case.
However, for the second one "I needn't have cooked dinner" we would either say:
Al final, no hacía falta hacer la cena.
or
Podía/Podría haberme ahorrado hacer la cena. 
This way you get the meaning of something you did but it wasn't necessary in the end.
I hope this helps.
Inma

Answer

Muchas gracias Inma.