"We did not have to listen."
Translation:Wij hoefden niet te luisteren.
It is actually usually the case- not to (clearly) pronounce the -n at the end of a word. I do not think it has to do with formality.
Perhaps, think of the English: Stolen, given. The -n is clearly pronounced.
Try to pronounce these words the same way but with a very soft -n. That is close to how to pronounce -en at end of a word is Dutch. (If that helps)