since引导的瞬间动词作连词。表“自从”
e.g.
1.She moved to London last May and has since got a job on a newspaper.
2. It was the first time I'd won since I'd learnt to play chess.
since作介词。放于句首或句中。表“既然”
e.g.
1.Since the lovely are sleeping,
2.Since you come, let's begin!