1. 概念：过去发生或已经完成的动作对现在造成的影响或结果，或从过去已经开始持续到现在并且有可能继续下去的动作或状态。

2. 时间状语：recently, lately, since, for, in the past few years等。

3. 基本结构：主语 have / has done 其他

否定形式：主语 have / has not done 其他

一般疑问句：Have / Has 主语 done 其他

4. since的三种用法：

（1） since 过去的一个时间点(如具体的年、月、日期、钟点、1980, last month, half past six)。

如：I have been here since 1989. 

（2） since 一段时间 ago。

如：I have been here since five months ago.

（3）since 从句（一般过去时）。

如：Great changes have taken place since you left.

比较since和for

since 用来说明动作起始时间，for用来说明动作延续时间的长度。

如：We haven’t had any guests since we moved in here.

I have lived here for more than twenty years.

注意：并非有for 作为时间状语的句子都用现在完成时。

I worked here for more than twenty years. (我现在已不在这里工作)