是before词义“在...时间之前”,而since词义是“自从.时间以来”,until是“一直到.时间”,显然,后两个词的词义是不合适的.那如果前面是过去式用什么？如果是完成时用什么呢？It was ten years before he came back.It had been ten years until/ before he came back.since在什么情况下用呀It has been ten years since he came back.It is ten years since he came back.since表示从过去一个时间点到现在，所以句中应该是现在时态，现在完成时和一般现在时都属于现在时态。