Wordsmith Wednesday: Since when?

It's been a while since I've done a new Wordsmith Wednesday feature. Now say that in French: for "since" do we use depuis or il y a? Both phrases denote passage of time, so it 's confusing -- so confusing that I spent a half hour on it in one-on-one immersion class in Provence, without benefit of English examples.

I did (sort of) learn it, however, and since we're under no such no-English-in-this-room-buster constrictions here, let me try to explain the difference, because this is apparently a big deal to the French, and the first question you're likely to be asked by a Frenchman is either "How long have you been here?"  or "How long have you not been (not) speaking French?"

Depuis means both "for" and "since," as in "I've been in France for ten days" - Je suis en France depuis dix jours and "I've been sick since Tuesday" - Je suis malade depuis mardi. 

However, if you want to say, "I arrived an hour ago," you use il y a. So it's Je suis arrivée il y a une heure, which gets confusing, because the English literal construction of that seems to be "It's been an hour since I arrived" and we just said that "since" translates to depuis.

What my French teacher told me -- and how she managed to do this all in French I can't recall, but it's quite a feat -- is that if the action is ongoing, you generally use depuis. If it's something in the past, use il y a. Doesn't always work, but close enough.

If you're determined to get this right (and your effort is probably better spent elsewhere), check out the fine page on this at french.about.com.