since引导时间状语从句,意为“从……时候”“自……以来”,主句要用完成时态,但时间的计算方式根据动词表示非延续性还是延续性而略有不同。
    a. since后如果是非延续性动词的过去式,则时间的起点应从该动作发生时算起。如：
    Mr.Li has been here since he came back.自从李先生回来以后,他一直在这儿。
    I have written home but once since I came here.自我到这儿以来,只给家里写过一封信。
    b. since后如果是延续性动词的过去式,则时间的起点应从该动作结束时算起。如：
    He has written to me frequently since he was ill.自从他病好以来,经常给我来信。
    c. It＋be＋some time＋since短语或从句(如果从句中谓语动词是非延续性动词,意为“做某事已有多长时间”；如果是延续性动词,意为“不做某事已有多长时间”)。注意主句与从句时态的一致性。如：
    It is(has been)a long time since the last meeting.自上次会议到现在已有很长时间了。
    It is two years since I smoked.我戒烟已有两年了。
    It has been two years since I began to smoke.我吸烟已有两年了。
    It's two years since he was in the army.他离开军队两年了。
    d. 有时时间的起点计算,主要根据句子的意思来考虑。如：
    I've known Mr.Smith since I was a boy.我小的时候就认识史密斯先生了。