Query re tense used for sentences with 'depuis'

Question

I am confused about an answer.  The sentence 'Tu n'as pas habité ici depuis long temps' was marked incorrect and ´Tu n'habites pas ici depuis long temps' given as the correct answer. But in the corresponding lesson we are told that in an affirmative sentence with 'depuis' you use present tense but if it is a negative sentence, you use passé composé.  Where am I going wrong?

Accepted Answer

Your 2 example sentences are directly linked to the lesson - see the section on special cases of depuis longtemps (I have added the bracketed/bolded comments): 
 
For "Tu n'habites pas ici depuis long temps’
Je ne vis pas en France depuis longtemps.
 
I haven't been living in France for long.  (But I am now)
 
 
Nous n'habitons pas ici depuis très longtemps.
 
We haven't lived here very long.  (But they are now)
 
 
 
For "Tu n'as pas habité ici depuis long temps”
Je n'ai pas vécu en France depuis longtemps.
I haven't lived in France in a long time. (And you are not now)
 
 
 
Chris, Alan is correct in his comments.

Answer

It's not just about negation or not that decides the tense used. It depends on whether the action is still ongoing in the present, too.

Tu n'habites pas ici depuis longtemps. -- You haven't been living here for long (and you still don't live here).
Tu n'as pas habité ici depuis longtemps. -- You haven't lived here for long (but now you do).

Answer

I think what you've missed, Ulrike, is that there is a section of the lesson specifically about "longtemps". The point is that sometimes the negation affects the verb (i.e. you haven't done something in a long time) and sometimes the word "longtemps" (i.e. you're doing it now, but not for long).

@Chris - the English translations in your examples are not correct, I think.