1)since+過去一個時間點(如具體的年、月、日期、鐘點、1980,lastmonth,halfpastsix)。

Ihavebeenheresince1989.

2)since+一段時間+ago

Ihavebeenheresincefivemonthsago.

3)since+從句

Greatchangeshavetakenplacesinceyouleft.

Considerabletimehaselapsedsincewehavebeenhere.

4)Itis+一段時間+since從句

ItistwoyearssinceIbecameapostgraduatestudent.

Since用來說明動作起始時間，for用來說明動作延續(xù)時間長度。

Ihavelivedhereformorethantwentyyears.

IhavelivedheresinceIwasborn..

Myaunthasworkedinaclinicsince1949.

Somenewoilfieldshavebeenopenedupsince1976.

IhaveknownXiaoLisinceshewasalittlegirl.

MybrotherhasbeenintheYouthLeaguefortwoyears.

Ihavenotheardfrommyuncleforalongtime.

注意：并非有for作為時間狀語的句子都用現(xiàn)在完成時。

Iworkedhereformorethantwentyyears.

(我現(xiàn)在已不在這里工作。)

Ihaveworkedhereformanyyears.

(現(xiàn)在我仍在這里工作。)

小竅門：當(dāng)現(xiàn)在完成時+一段時間，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)動詞在完成時中的誤使。

1)(對)TomhasstudiedRussianforthreeyears.

=TombegantostudyRussianthreeyearsago,andisstillstudyingitnow.

2)(錯)Harryhasgotmarriedforsixyears.

=Harrybegantogetmarriedsixyearsago,andisstillgettingmarriednow.

顯然，第二句不對，它應(yīng)改為Harrygotmarriedsixyearsago.或Harryhasbeenmarriedforsixyears.