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。
