1)since+过去一个时间点(如具体的年、月、日期、钟点、1980,lastmonth,halfpastsix)。
Ihavebeenheresince1989.
2)since+一段时间+ago
Ihavebeenheresincefivemonthsago.
3)since+从句
Greatchangeshavetakenplacesinceyouleft.
Considerab letimehaselapsedsincewehavebeenhere.
4)Itis+一段时间+since从句
ItistwoyearssinceIbecameapostgradua testudent.
Since用来说明动作起始时间，for用来说明动作延续时间长度。
Ihavelivedhereformorethantwentyyears.
IhavelivedheresinceIwasborn..
Myaunthasworkedinaclinicsince1949.
Somenewoilfieldshavebeenopenedupsince1976.
IhaveknownXiaoLisinceshewasalittlegirl.
MybrotherhasbeenintheYouthLeaguefortwoyears.
Ihavenotheardfrommyuncleforalongtime.
注意：并非有for作为时间状语的句子都用现在完成时。
Iworkedhereformorethantwentyyears.
(我现在已不在这里工作。)
Ihaveworkedhereformanyyears.
(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)TomhasstudiedRussianforthreeyears.
=TombegantostudyRussianthreeyearsago,andisstillstudyingitnow.
2)(错)Harryhasgotmarriedforsixyears.
=Harrybegantogetmarriedsixyearsago,andisstillgettingmarriednow.
显然，第二句不对，它应改为Harrygotmarriedsixyearsago.或Harryhasbeenmarriedforsixyears.
