现在完成时

现在完成时表示过去的动作已经完成，但对现在的情况仍存在一定的影响；也可以表示一致持续到现在的动作或状态。

常用的时间状语为：recently、lately、since...for...、in the past few years、just、already、yet、up to now、till now、so far、these days等。

I have just finished my homework.
我刚刚完成家庭作业。

He has already got her help.
他已得到她的帮助。

现在完成时的一般形式：

肯定形式：have/has done;
否定形式：have/has + not + done;

一般疑问句形式：把have/has放于句首。

I've just finished my work now.
我刚完成工作。

It hasn't rained for half a year in that place.
那个地方已经半年没有下雨了。

Have you ever been to Beijing?
你去过北京吗？

现在完成时的扩展：

1）for + 时间段、since + 时间点。

I have lived here for five years.
我已经在这里住了5年了。

I have lived here since 1985.
我从1985年开始就住在这儿了。

如果句中含有表示“最近几世纪/年/月以来”的时间状语，例如：over the past few years、in the past few years/months/weeks/days、during the last three months、for the last few centuries、through centuries、throughout history等，谓语动词用现在完成时。

I have been a teacher over the past few years.
在过去的几年里我一直是个老师。