题目内容：

Four years()since I ()the army 求分析
为什么是 has passed joined
而不是 passed have joined

优质解答

Four years has passed since I joined the army .我参军四年了.
since 引导的时间状语从句,主句用现在完成时,从句用一般过去时表示.
eg .I have lived here since I was born .