Past tense

Question

I wrote: Qué metiste en esa caja? and was marked wrong. The right answer should be : Qué metias en esa caja?

As I understand it you did not put something in the box habitually. You did it at a special occasion. So what's wrong with "metiste"?

Accepted Answer

Hola Ole
¿Qué metiste en esa caja? is also grammatically correct if the translation we want is "What did you put in that box?", implying a precise moment/completed action.
But the translation that we give is one that implies an ongoing action in the past using "were you putting..." :
¿Qué metías en esa caja?  (What were you putting in that box?)
The hint is also saying "Conjugate in El Imperfecto" - This is why in this specific test the answer using El Indefinido is wrong. 
I hope this clarified it.
Saludos
Inma