比较since和forSince用来说明动作起始时间，for用来说明动作延续时间长度。例如：Ihavelivedhereformorethantwentyyears.我住在这儿二十多年了。IhavelivedheresinceIwasborn.我从出生起就住在这儿了。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。）注意：用句型转换的方法，很容易排除非延续动词在有for/since结构的完成时中的误用。1）（对）TomhasstudiedRussianforthreeyears.=TombegantostudyRussianthreeyearsago,andisstillstudyingitnow.2）（错）Harryhasgotmarriedforsixyears.=Harrybegantogetmarriedsixyearsago,andisstillgettingmarriednow.显然，第二句不对，它应改为Harrygotmarriedsixyearsago.或Harryhasbeenmarriedforsixyears.
