求It is three years since he worked in his office.解释下since
问题描述：
求It is three years since he worked in his office.解释下since
答
这是现在完成时since(从...起)的用法.相当于He has worked in his office for three years
he worked in his office three years ago
答
since------------------自从
答
“自从” since后面一般为过去式