表示肯定含义,意为“自从……以来”,例如:It is three years since she began to smoke. (她抽烟已经3年了);2.当since从句的谓语是延续性动词时,表示否定含义,意为“自从不……以来”,例如:It is three years since she smoked.(她戒烟已经3年了)...