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.