passe compose vs imparfait

Question

I think that the use of imparfait vs passe compose (sorry I don't know how to use accents on this keyboard) is a matter of opinion in several cases here.  For example, I used "achetait des croissants."  He did this regularly.  The answer is "a achete des croissants."  He did it once.  Either could be correct.  The same with "est devenu une nouvelle personne."  Did this happen at once or did it happen continuously?  Judging by some other comments, I am not the only one who thinks either tense could be used in some cases.

Accepted Answer

Bonjour Carl,
Le Passé Composé is used because ‘that day’ / ‘at that point’ (hints given) give a clear timeframe and report a change in his behaviour / state of being. All these actions are in Le Passé Composé as they are all completed and happened once.
‘croisait’ is in L’Imparfait because: see Jim’s answer
I hope this is helpful. 
Bonne journée !