注意，since和as可以引导时间状语从句，for和because是不行的。由since引导的时间状语从句。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（注意翻译）。例如：I have been in Beijing since you left.自从你离开后，我一直在北京。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 Shanghai.我妹妹不在上海住有四年了。...
