For和since都表示我們說(shuō)到的時(shí)間從過(guò)去開(kāi)始一直持續(xù)到現(xiàn)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(dāng)我們用for的時(shí)候，表示一個(gè)活動(dòng)持續(xù)一段時(shí)間，從過(guò)去到現(xiàn)在。
我們用since的時(shí)候，表示過(guò)去的行為開(kāi)始一直持續(xù)到現(xiàn)在。
兩句話有同樣的含義。for的例子中用了“13年”這個(gè)時(shí)間，但由于 2003年加上13年就是2016年，即今年。兩句話的重點(diǎn)都是現(xiàn)在，“我”繼續(xù)住在意大利，沒(méi)有去其他國(guó)家。

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。

給大家舉一些例子, 什么時(shí)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。)?

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.
?