"Do you know when Anna lost the keys?"
Translation:Ты знаешь, когда Анна потеряла ключи?
Why cannot we use "теряла" instead of "потеряла" ? this sentence was marked wrong : "ты знаешь когда анна теряла ключи".
My guess would be that потерять is the perfective version of терять, which is used when the action is finished - which seems the case here as the action takes place in the past.
However, I feel that marking the use of терять as wrong seems a little too harsh, so is it a limitation of Duolingo, or is there an actual good reason for refusing the imperfective version of the verb?
There's only one way to use perfective: It's used for single events where the event has been fully completed. It's true that the sentence doesn't give us enough information to know if the perfective aspect could be an option but with the information that they give us it's enough to use the perfective after all we're doing the perfective lessons.