il y a = ago
J'ai commencé à apprendre le français il y a longtemps.
I started learning French a long time ago.
Je suis allé en France il y a 6 moins.
I went to France 6 months ago.
depuis = since/for (where it continues to the present)
J'apprendre le français depuis longtemps.
I have been learning french for a long time (and still am).
Je travaille depuis 6 heures ce matin.
I have been working since 6 o'clock this morning.
pendant = for (with no relation to the present)
J'ai étudié le français pendant 2 ans.
I studied french for 2 years (but don't anymore).
J'ai travaillé pendant 11 heures aujourd'hui.
(I worked for 11 hours today.)
I'm not a native French speaker though, so it's entirely possible that I'm way out!