句中before引导时间状语从句.如：
It was three years before he knew it.不知不觉中3年过去了.（三年后他才知道）.
It is（was/has been)...since...职工,时间计算一律从since从句的动作完成或状态结束时算起,如：
It's three years since he lived here.
他不在这儿住有三年了.
It's three years since I smoked.
我不吸烟有三年了.
It’s three years sinceI began to smoke.
我吸烟有三年了.