现在完成时重难点

① 由since引导的时间状语，它们的主句通常使用完成时态。

He has taught English since 1970.

It has been a long time since I last saw you.

② 只有在It is …since…这种结构中，主句的谓语才可用一般现在时，现在完成时或一般过去时。如：

It’s ten years since I left school.

It’s a long time since I saw you last.

③ 表示短暂性的动词不能与表示一段时间的状语连用，这类动词有：

begin,  borrow,  buy,  close,  come,  die,  fall,  find,  finish,

join,   kill,     lend,  leave,  sell,   start,  stop等，如：

不能说：His father has died for three years.

只能说：His father died three years ago.

不能说：He has left home for two months.

只能说：He left home two months ago.

④ 非延续性动词的否定形式可以表示状态的延续。可与for, since时间状语连用。如：

I haven’t seen him for a long time.

I haven’t heard from him since he left.

She hasn’t left home ever since she lost her job.

⑤ have(has) been 和have (has) gone的区别。

Have been to a place意思是“到过、去过”，表示曾到过某处，但现在人不在那儿；have gone to意思是“去了”，表示已经去了某地，现在人可能在去的途中或已在那儿。如：

Has she ever been to Nanjing ?

You have never been there before, have you?

I have been to Guilin, I went there last year.

总之，have been to讲的是过去的情况，强调去过或没有，着重于到目前为止的一个结果，而have gone to指现在人在不在，只用于第三人称，不用于第一、第二人称，不能用来代替have been to.