現在完成時for和since的用法
For和since都表示我們說到的時間從過去開始一直持續到現在，但是for與since不能混用。
例如：
“I have lived in Italy for 13 years.”
“I have lived in Italy since 2003.”
當我們用for的時候，表示一個活動持續一段時間，從過去到現在。
我們用since的時候，表示過去的行爲開始一直持續到現在。
兩句話有同樣的含義。for的例子中用了“13年”這個時間，但由於 2003年加上13年就是2016年，即今年。兩句話的重點都是現在，“我”繼續住在意大利，沒有去其他國家。
Since 用來說明動作起始時間，for用來說明動作延續時間長度。
給大家舉一些例子, 什麼時候該用for和since
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.
但是並非有for 作爲時間狀語的句子都用現在完成時。
I worked here for more than twenty years.
(我現在已不在這裏工作。)
I have worked here for many years.
(現在我仍在這裏工作。)
當現在完成時+一段時間，這一結構中，我們用下面的公式轉化，很容易就能排除非延續動詞在完成時中的誤使。
1) (對) Tom has studied Russian for three years.
= Tom began to study Russian three years ago, and is still studying it now.
2) (錯) 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，它應改爲 Harry got married six years ago. 或 Harry has been married for six years.
