I don't understand why we can't use the Passé Composé

Bonjour Ana K

"during that year the letters had been sent"

Without reference to a particular kwiziq lesson or story where there may have been some further context, I see this as an activity that used to be the case.

That suggests to me a report of a background historic situation only which is why it is in the imperfect.

This is why regretfully I am unable to agree with your proposal.

Bonne journée

Jim

Hi Ana, 

To add to what Jim has said, without the specific context, it is not possible to give a clear answer.

Could you point us to the specific exercise where you saw that sentence?