Hi Richard,

Here is my take on it. It is quite tricky and nuanced.

When referring to time periods, depuis can be used both to indicate ‘time since when’ :

Il est malade depuis vendredi – He has been sick since Friday

as well as, ‘period since the beginning of which’ :

il est malade depuis un an – he has been sick for a year

The tenses used with depuis are tricky and differ from those used in similar constructions in English:

I have lived (and still do) in Paris for ten years – J’habite à Paris depuis dix ans

I (had) lived (but no longer) in Paris for ten years – J’habtais à Paris depuis dix ans

Where English uses the perfect French uses the present; where English uses the pluperfect or preterite, French uses the imperfect.

When using depuis in negative constructions, the passé composé (perfect) as well as the present can be used; it depends on the context.

Je ne dors pas depuis deux jours – I have not slept for two days (and I’m still not able to sleep)

Je n’ai pas dormi depuis deux jours – I have not slept for two days (but now I’m ready to sleep)

Depuis can also be used in the sense of from when referring to place or time:

Le Canada s’étend depuis l’Atlantique jusqu’au Pacifique – Canada stretches from the Atlantic to the Pacific.

Depuis lundi jusqu’à vendredi – From Monday to Friday

Depuis que (+ conjugated verb) : this construction has two meanings:

(i) it marks a point in time and can be translated as since or after:

Je ne la vois plus depuis qu’elle a deménagé. – I don’t see her any more since (after) she moved

This construction uses the same tenses as the English equivalent.

(ii) it can express duration which is on-going and not solely fixed to a point in time :

Je l’aime aimais depuis qu’elle est arrivée à Paris – I have loved her since she arrived in Paris

Note in this case French uses the present imperfect while English uses the perfect.

I hope this helps and is not too confusing.

Tom, thanks for the detailed discussion. 

I think that the last example is incorrect though as it should be - «Je l'aime depuis qu'elle est arrivée à Paris.»  Present tense is needed here as the 'loving' is ongoing.  Using the present tense (Le Présent) - and not the compound past (Le Passé Composé) - in sentences with "depuis" (since/for) in French (French Prepositions of Time)