Since用來說明動作起始時間，for用來說明動作延續時間長度。
Ihavelivedhereformorethantwentyyears.
IhavelivedheresinceIwasborn..
Myaunthasworkedinaclinicsince1949.
Somenewoilfieldshavebeenopenedupsince1976.
IhaveknownXiaoLisinceshewasalittlegirl.
MybrotherhasbeenintheYouthLeaguefortwoyears.
Ihavenotheardfrommyuncleforalongtime.
注意：並非有for作為時間狀語的句子都用現在完成時。
Iworkedhereformorethantwentyyears.
(我現在已不在這裡工作。)
Ihaveworkedhereformanyyears.
(現在我仍在這裡工作。)
小竅門： 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)TomhasstudiedRussianforthreeyears.
=TombegantostudyRussianthreeyearsago,andisstillstudyingitnow.
2) (錯)Harryhasgotmarriedforsixyears.
=Harrybegantogetmarriedsixyearsago,andisstillgettingmarriednow.
顯然，第二句不對，它應改為Harrygotmarriedsixyearsago. 或Harryhasbeenmarriedforsixyears.