谁能帮我全面的解释一下since这个词

Since(基本用法如下)

Ⅰ. Conj.从…以来;…以后:I haven’t seen her since she left.
由于;既然: Since you aren’t interested, I won’t tell you about it.
Ⅱ. prep. 从…以来;自从(常和完成时连用): I haven’t heard from her since last year.
Ⅲ. adv. 从那时以后;后来(与完成时连用): She moved to London last May and has since gotten a job on a newspaper.

现就你提出的具体问题作出解答,如果没有理解错你应该是问这个句型it is/was +时间段n.+ since(自从...已经多久了)
关于这个句型要注意以下几个方面(时态搭配和辨别v.是延续性v.还是短暂性v.)
1.it is +时间段n.+ since sb. have done(该v.必须为延续性v.,翻译时直译)
或是it is +时间段n.+ since sb.did(如v.为短暂性v.即瞬间性v.翻译时直译,如为延续性v. 翻译时反译)
eg.It is 5 years since I have lived here.(live为延续性v.故直译为自从我住在这儿已经5年了。)
It is 5 years since I lived here.（live为延续性v.故反译为我已经5年不住这儿了。）
It is 3 years since I smoked.（smoke为延续性v.故反译为我已经3年不吸烟了。）
It is 3 days since the accident happened.（happen为短暂性v.故直译为自从这场车祸发生已经有3天了。）
It is 10 years since they married.（marry为短暂性v.故直译为自从他们结婚已经有10年了。）
2.it was +时间段n.+ since sb. had done（就实际情况翻译）
He said it