since和for的用法：
    since 用來說明動作起始時間，for用來說明動作延續(xù)時間長度;
    eg：I have lived here for more than twenty years.
    I have lived here since I was born.
    for后面多接一段時間，而since后面則接一個時間點，若since后面也出現(xiàn)時間段，則在時間段后加ago;
    eg： I have lived here for twenty years ago.
    錯句：I have worked here since twenty years.
    since句型：
    It is +一段時間+ since從句，since從句要用一般過去時;
    eg：It is two years since I became a postgraduate student.
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