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：我已经在这儿住了二十多年了
I have lived here since I was born：我从出生就住在这里
My aunt has worked in a clinic since 1949：我的阿姨自1949以来在诊所工作
Some new oilfields have been opened up since 1976：一些新的油田自1976年已开放
I have known Xiao Li since she was a little girl：我从小李是个小女孩时就认识她
My brother has been in the Youth League for two years：我的哥哥入团已经两年了
I have not heard from my uncle for a long time：我已经很长时间没有收到我叔叔的来信了
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性动词在完成时中的误使。
1) (对) Tom has studied Russian for three years.
= Tom began to study Russian thre e 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.
