didn't need vs needn't have

Question

¡Hola!
didn't need to (infinitive) & needn't have (past participle) are used to express the lack of necessity in the past, however
didn't need implies that the speaker didn't do something because he/she new that it was not necessary
needn't have means the speaker did something and then he/she knew that it had not been necessary
for example:
I didn't need to have an interview because I had worked there before
I needn't have cooked dinner. Just as it was ready, Chris and June phoned to say that they couldn't come to eat
(examples are taken from Advanced Grammar in Use by Martin Hewings)
How can I express it in Spanish?
Regards,
Alexander

Accepted Answer

Hi Alexander
Sorry about this huge delay, I wasn't notified about your extra question in October.
For "I needn't have cooked dinner [as there was no need in the end] you can indeed say "no tenía que haberlo hecho" with the same meaning as the ones I suggested before. Or "no era necesario hacerlo".
Saludos
Inma

Answer

Hola Alexander,
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

Hola Inma)
Is it possible to say "no tenía que haberlo hecho" or " no haberlo hecho"?
Regards,
Alexander

Answer

¡Muchas gracias, Inma!