present perfect 现在完成时.就是你已经做了某些事情了.
时态标志词主要有：for,since,so far,ever,never,just,yet,till / until,up to now,before,already,recently,in recent years,in the past few years
如：
I haven’t seen a student like that before.我以前没有见过这样的学生.
They have just finished lunch.他们刚吃完午饭.
The bus has already left.公车已经开走了.
Have you heard from him yet?你收到他的来信了吗?
Have you ever been to Los Angles?你去过洛杉矶吗?
2.现在完成时与for/since等引导的持续性时间状语或so far等包括“现在时间”在内的时间状语连用.
3 by +时间点,表示“到…为止”,要和完成时态连用.当叙述过去的事情时,要用过去完成时.