三个词引导时间状语从句时我区别. before ,until 这两个词用一般现在时表示将来时.如 I will wait here until he comes back. It will be two years before he returns. 当主句是过去时的时候从句也用过去时.如 Yesterday I waited until he came back. He glanced at the watch before he left. since 有它自己的用法,既它的从句用一般过去时,主句用现在完成时. I have worked here since I came here two years ago.
