4-2现在完成进行式(Present Perfect Progressive Tense)
Since 1974, I have studied English at school.
Since I graduated from college, I have worked here.
以第一句话为例，假如我们要强调我一直在练习游泳，而且没有间断，我们可以用现在完成进行式(present perfect progressive tense)。
所谓现在完成进行式，形式：verb to have + been + present participle
verb to have 是为了完成式，been 和 present participle 都是为了进行式。
I have been studying English since 1974.
I have been working here since I graduated from college.
I have been living here since I was a child.
He has been acting like a fool lately.(他最近一直在做傻事)
They have been dancing since seven o’clock.
It has been snowing since yesterday.
I have been taking music lessons since last year.
He has been drinking heavily since last year.(他去年起，就一直在酗酒)