解放军文职考试：由since引导的时间状语从句

　　since引导的从句的谓语动词可以是延续性的动词，又可以是瞬时动词。一般情况下，从句谓语动词用一般过去时，而主句的谓语动词用现在完成时。但在It is +时间+since从句的句型中，主句多用一般现在时。例如：

　　I have been in Beijing since you left. 自从你离开以来，我一直在北京了。

　　Where have you been since I last saw you? 自上次我和你见面以后，你到哪里去了?

　　It is four years since my sister lived in Beijing. 我妹妹不在北京住有四年了。

　　It is five months since our boss was in Beijing.我们老板离开北京有五个月了。