Pete wasn't studying very hard when he was at university [1].
The instruction of the question on the exercise I was doing is determining which one sentence is incorrect. The key answer says the sentence above is incorrect, but it doesn't explain why.
Don't we use past progressive for repeated actions around a particular time? For instance, the example I took from Oxford Grammar:
I was playing a lot of tennis when I got to know Peter [2].
The particular time of [1] is when Pete was at university and [2] is when the speaker got to know Peter. We don't know how long the speaker had got to know Peter in [2], as well as we don't know how long he had studied at university in [1]. Then why is the sentence [1] incorrect?