Definitions
1.AdverbFrom a specified time in the past."I met him last year, but haven't seen him since.; A short/long time since"
2.PrepositionFrom: referring to a period of time ending in the present and defining it by the point in time at which it started, or the period in which its starting point occurred.
Continuously during that period of time."I'd known her only since the previous year, so from/since the moment we got married we’ve quarrelled.; I'd been working since six o’clock, and I was getting tired."
3.PrepositionContinuously during that period of time."I'd known her only since the previous year, so from/since the moment we got married we’ve quarrelled.; I'd been working since six o’clock, and I was getting tired."
4.PrepositionAt certain points during that period of time.
5.ConjunctionFrom the time that."I have loved you since I first met you.; I've known her since I've lived in her street, but I've not seen her out of the house since her husband died.; Since leaving school, I've completely changed."
6.ConjunctionBecause."Since you didn't call, we left without you."
7.ConjunctionWhen or that.