since 用來(lái)說(shuō)明動(dòng)作起始時(shí)間，for用來(lái)說(shuō)明動(dòng)作延續(xù)時(shí)間長(zhǎng)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 作為時(shí)間狀語(yǔ)的句子都用現(xiàn)在完成時(shí)。

　　i worked here for more than twenty years.

　　(我現(xiàn)在已不在這里工作。)

　　i have worked here for many years.

　　(現(xiàn)在我仍在這里工作。)

　　小竅門(mén)：　當(dāng)現(xiàn)在完成時(shí)+一段時(shí)間，這一結(jié)構(gòu)中，我們用下面的公式轉(zhuǎn)化，很容易就能排除非延續(xù)動(dòng)詞在完成時(shí)中的誤使。

　　1)　(對(duì)) tom has studied russian for three years.

　　= tom began to study russian three years ago, and is still studying it now.

　　2)　(錯(cuò)) harry has got married for six years.

　　= harry began to get married six years ago, and is still getting married now.

　　顯然，第二句不對(duì)，它應(yīng)改為 harry got married six years ago.　或 harry has been married for six years.