请问for year与since year 与year ago的用 法区别

since和for的用法与区别
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.