同学你好，该知识点来自沪江网校《英语零基础直达大学六级【全额奖学金班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好~

若since引导的状语从句的谓语动词是终止性的过去式时，则从句表示的时间是“从动作开始的那一时刻起”。
如：  
He has studied very hard since he came to our school.自从他来到我们学校，他学习就非常努力。   We have been missing them since they left here. 自从他们离开这里，我们就一直很想念他们。

若since引导的状语从句的谓语动词是持续性动词或表示状态的动词的过去式时，则从句表示的时间是“从那持续动作或状态结束时算起”。
如：
John is now with his parents in New York, it is already three years since he was a teacher.
约翰现在和父母一起住在纽约，他不当教师已经三年了。

题目：-- It seems that his health is improving.
-- Yes, it is ten years _______ he _______.
选项：1.after; didn't smoke
2.before; smoked
3.since; smoked
4.that; smoked
答案：3

这里smoke是延续性动词，所以要指他不抽烟了，应该是用肯定形式，就指的是从动作结束的时候。

祝同学学习进步~