using negative sentences with depuis and using passé composé

Question

I am confused by this example:

"I haven't left France for three years:" "Je n'ai pas quitté la France depuis trois ans."

The lesson says "we use PC because the negation indicates the action wasn't done during the entirety of that past period. BUT PC is used to express actions which were completed or finished in the past.

This example shows it WAS NOT completed, so why PC and not imparfait??

Thank you.

Accepted Answer

Laura,
 I have added a link below to a related question. Both passé composé and imparfait can be and are used for past completed events.
For events/happenings from past and still happening in present, passé composé is used, not imparfait. 
The links referred to in the attached answer are excellent discussions of the practical use of passé composè or imparfait by native French speakers.
 https://progress.lawlessfrench.com/questions/view/i-ve-always-wanted-to-try

Answer

As Maarten says, the choice of passé composé vs imparfait for longer duration actions is to use passé composé for those NOT completed in the past whereas imparfait is used for those that have completed. 
The use of present tense with depuis for actions that started in the past and are still ongoing in the present is because in French you formulate the sentence looking on the event from the present, where it is still happening. This, to French speakers, mandates the use of the present tense. In English, you look at it from the past, where it started. That's why in English you mostly use the present perfect or past tenses. But, remember, that's for actions that are STILL ONGOING.
Using depuis with negative sentences is a bit different. Just remember to use the passé composé, which will be correct in most cases. The logic behind it is that, in your example, the action of not having left France is still ongoing in the present and hence demands the passé composé.