I agree that the suggested translation uses the English preterit tense. If there was more context to show that this past action didn't end, then it would be imperfect. For instance, "What was left to do before we became distracted?" provides enough context to show the past action wasn't completed
I think your sentence could be translated as "Cos'ha lasciato fare" The given sentence literally means what was remaining to do, so it's a little different. Using active wouldn't sound right (What remained to do) so instead passive is used (what Was remaining/left to do). Hope I helped!
In English "did leave" is one of two ways of expressing the preterit tense, which is a past tense. The other way is to say "left". The usage of "did + infinitive" is typically used in questions, negations, and emphatic sentences. For instance, "Did you leave the keys in the car?", "I did not leave the keys in the car", "Oh no... I really did leave them in the car." When speaking in the preterit tense, we are talking about a past event that happened once and is over with
However, there is another indicative past tense, the imperfect, which we are practicing here. The imperfect tense is about a past event that was happening in some sort of ongoing, continuous, or repeated manner. Typically in English we express this sense of an ongoing action in the past with several different constructions: "used to + infinitive", or "was/were + present participle", or the preterit tense of the word used in conjuction with some modifying word that makes it clear that the action happened repeatedly, or habitually. For example, "You were leaving your keys in the car until your car was finally stolen." "You habitually left your keys in the car." "Because you used to leave your keys in your car, you made a new year's resolution to be more careful."