现在完成时语法现在完成时already,ever,for,just,never,since,yet的用法

现在完成时表示动作过去开始,但持续到现在,或过去发生的动作,对现在产生后果或影响.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.
注：表示短暂行为的动词不能和延续性时间状语连用.
如:I have been back from England for over two weeks.或It has been over two weeks since he came back from England.