Mientras - Using preterite or imperfect

Question

The correct answer here shows both verbs in imperfect tense. Why is using preterite in the first clause and imperfect in the second clause incorrect? The lesson shows both examples in the past tense.

Silvia Piriz · Accepted Answer

Hola Linda G.
Thanks for your question! Could you let us know the specific example from the lesson that you’re referring to? That way, we can check exactly why the answer is shown with both verbs in the imperfect tense, and whether using the preterite in the first clause would change the meaning.
Once we see the example, we’ll be able to explain the difference more clearly.
Hasta pronto
Silvia