I have a gap-fill exercise in which begin and start are given:
_By the time I got to the cinema, the film (1)____.
_ As soon as they (2)_____ to explain, other people told at them to be quiet.
The answers are (1)had started and (2)began. Why aren’t they (1)had begun and (2)started? Although I sense that the former is more natural, I just can’t explain why the latter is wrong.
Although there was a discussion on this, I didn’t find any suitable explanation for my case.