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
