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.