Please take a look at this link for the entry since, taken from The American Heritage Dictionary of the English Language (Fourth Edition). There, look carefully at the different meanings that the word since may have.
Originally Posted by Unregistered
When the word since here means from then until now or between then and now:
He's been unhappy since his girlfriend left him. The word since may also mean because. When the word since means because:
He's been unhappy ever since his girlfriend left him. (Here the word ever provides emphasis.)
He's been unhappy since his girlfriend left him. Here the meaning is different. One could only know the difference (i.e., whether 'from then until now or between then and now' or because is intended) from the context that the speaker provides.
And reversing the clauses won't help much either:
Since his girlfriend left him, he's been unhappy.Here since could still mean either 'from then until now or between then and now' or because. Greater context is still needed to differentiate the two meanings:
Now he's unhappy, since his girlfriend left him. (-Please note the comma.) The addition of the word now makes everything clearer here. In both of the two previous sentences, since now means because.
Since his girlfriend left him, now he's unhappy. (-ditto)
Hope this puts to rest your confusion. -Isn't English fun?