i am not going to say since there are so many replies.

  在It is +时间+since从句的句型中,主句多用一般现在时。It’s … since …句型中的It指的是时间, 可以说It has been … since …, 但习惯上常说It’s … since …。例如:

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。

It’s half an hour since the film began。

电影开始有半小时了。

It’s an hour since he fell asleep。

他睡着有一个小时了。

It’s three years since his father died。

他父亲去世有三年了。

It’s two months since he left home。

他离家已有两个月了。

It’s three days since he came back。

他回来已三天了。

相对而言,before,after的用法要简单一些:

It had been 5 years before I realized my dream。

  (用过去完成时)=It took me 5 years to realize my dream。为实现梦想,我花去了5年时间。

It had been 5 years after I came back from Britain。总觉得这里after好象不符合语言习惯,换成since将更通顺一些。

  Since一般是与现在完成时连用的,为自。。。以来。例如,he has lived here since five years ago。

Before表示从过去某一时刻算起的以前。一般也为完成时。例如,He told me that the trian had left 5 minutes before。

  当它作为介词,是在。。。之前;作连词,后面跟从句。还有来不及。。。就。。。例如,she had left beroe I could say a fore引导的时间状语从句,表示主句的动作发生在从句的动作之前。主句和从句中的动词都用过去时。

为了表明主句的动作发生或完成在从句动作之前,主句中的动词一般用过去完成时,从句中动词用过去时,以区分动作的先后。

before引出的从句不可用否定式谓语,尽管汉语可以说“趁你还没忘”,但在英语中仍为before you forget。

连词after表示“在。

  。。之后”,既主句的动作发生在从句动作之后。after引导的时间状语从句的句子可以和before引导的时间状语从句相互转换。