Stop insisting that we use "used to" to translate every sentence that is in the imperfect! That isn't how English works.
"we used to write each other love letters secretly" is marked wrong, why?
We were writing each other love letters in secret Marked wrong (But this is how people actually talk )
Exactly - any decent editor would delete the "used to"; it is just extraneous verbiage. An English speaker or writer would say "wrote." Does Duo have any native English speakers?
"We wrote some love letters to each other in secret." marked wrong and reported.