"The party continues since last night."
Translation:La fiesta sigue desde anoche.
I found a good Youtube video from Lightspeed Spanish here: http://www.youtube.com/watch?v=s0pk_2Nyq2c It's ten minutes long and they speak with castillian accents (so "hace" sounds like "hathe,") mixed with a clear Scottish accent in the English parts. It's good.
Their take, which I have tried to break down: Hacer = "to do" or "to make," but is very versatile in different uses. For these examples: Hace = "ago" or in clumsy English "it makes."
Desde hace = "for the last" (Desde hace + a time (dos semanas, etc.)
Desde = since (Desde + a date (2004, or June 16th, etc.)
Basically,, in answer to a question like "How long have you worked here, you could say: 1)Desde hace: I have worked here for the last 10 years." = Llevo aqui desde hace diez años. "Desde hace" = "for the last" or "for."
(Not to complicate things, but if "desde hace" was translated to simply "since" here, it would make for clumsy English: "I have worked here since ten years." That's what makes for a crucial difference between "hace" and 'desde hace.")
2) Hace: "It makes ten years since I worked here," which is clumsy English. Looser but better: I have worked here 10 years ago." = Hace diez años que trabajo de aqui. " (In English, of course, we put the "ago part at the end; in Spanish it goes at the beginning.)
3) Desde: I have worked here since = Trabajo aqui desde 2004. = Trabajo aqui desde 2004. "Desde" = "since."
Since all of this is complicated, the Speed Spanish people suggest getting a handle on one of them at a time. If you are asked a question, you can always manipulate the answer to fit what you know.
The difference in past and present between Spanish and English in the use of words like "since" is similar, I think, to some French usage. For example, "She has been blond since she was young." might be translated (to French) in the present.
All of this is my take on it, but — Llevo lo siento por los errores. Soy sólo un estudiante también! Those who know more, please help if I've gone horribly astray.