现在完成时

现在完成时表示过去开始，但持续到现在的动作，或过去发生的动作，对现在留下后果或影响。常和for+一段时间，already, ever, never, just, yet, in the past few years/ over the last thirty years, so far, up to now, since 1980等状语连用。
【例如】
Most college students have learned English for more than six years. Great changes have taken place in China in the past few years.I have never been abroad up to now.*表示短暂行为的动词不能和延续性时间状语连用。
【例如】
I have been back from England for over two weeks.或It has been over two weeks since he came back from England.We couldn't say: I have come back from England for over two weeks.He has been in the army since 1998. We couldn't say: He has joined the army since 1998.