比较since和for_语法技巧
since 用来说明动作起始时间，for用来说明动作延续时间长度。
i have lived here for more than twenty years.
i have lived here since i was born..
my aunt has worked in a clinic since 1949.
some new oilfields have been opened up since 1976.
i have known xiao li since she was a little girl.
my brother has been in the youth league for two years.
i have not heard from my uncle for a long time.
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。)
i have worked here for many years.
(现在我仍在这里工作。)
小窍门： 当现在完成时+一段时间，这一结构中，我们用下面的公式转化，很容易就能排除非延续动词在完成时中的误使。
1) (对) tom has studied russian for three years.
= tom began to study russian three years ago, and is still studying it now.
2) (错) harry has got married for six years.
= harry began to get married six years ago, and is still getting married now.
显然，第二句不对，它应改为 harry got married six years ago. 或 harry has been married for six years.
i have lived here for more than twenty years.
i have lived here since i was born..
my aunt has worked in a clinic since 1949.
some new oilfields have been opened up since 1976.
i have known xiao li since she was a little girl.
my brother has been in the youth league for two years.
i have not heard from my uncle for a long time.
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。)
i have worked here for many years.
(现在我仍在这里工作。)
小窍门： 当现在完成时+一段时间，这一结构中，我们用下面的公式转化，很容易就能排除非延续动词在完成时中的误使。
1) (对) tom has studied russian for three years.
= tom began to study russian three years ago, and is still studying it now.
2) (错) harry has got married for six years.
= harry began to get married six years ago, and is still getting married now.
显然，第二句不对，它应改为 harry got married six years ago. 或 harry has been married for six years.