请看2003年高考英语试题(上海卷)第35题:

　　A good storyteller must be able to hold his listeners' curiosity he reaches the end of the story.

　　A. whenB. unlessC. afterD. until

　　析:答案是D。不少同学可能会误选unless,引导条件状语从句,但析语境可知,该空表"直到",强调时间的延续,应用until引导时间状语从句。

　　本题着重考查unless和until的用法区别,其用法区别是高考热点考点。本文拟谈一谈unless和until的用法区别。

　　一、作为从属连词含义不同,引导从句性质不同

　　unless与until均为从属连词,unless表"除非",强调条件,引导条件状语从句;until表"直到",强调时间的延续,引导时间状语从句。

　　1. I didn't know the news until my teacher told it to me.

　　2. I couldn't know the news unless my teacher told it to me.

　　3.-Was his father very strict with him when he was at school?

　　-Yes. He had never praised him until he became one of the top students in his grade.

　　析:句1动词didn't know表明该句强调时间的延续,表"我不知道这消息"的状态一直延续到"老师告诉我",因此应用until引导时间状语从句。句2动词couldn't know表明该句强调条件,表"我不可能知道这消息,除非老师告诉我",因此应用unless引导条件状语从句。句3表明"他爸爸从来没有表扬他"的状态一直延续到"他变成年级最好的学生之一",因此也应用until引导时间状语从句。

　　二、作为从属连词引导的从句常用一般现在时表示将来

　　unless引导的条件状语从句和until引导的时间状语从句常用一般现在时表示将来。

　　例1:除非邀请你否则别进来。

　　误:Don't come in unless you will be invited to.

　　正:Don't come in unless you're invited to.

　　例2:除非他来,我们不可能去。

　　误:Unless he will come, we won't be able to go.

　　正:Unless he comes, we won't be able to go.

　　三、作为从属连词引导的状语从句可转换为省略式创新句型

　　unless引导条件状语从句和until引导时间状语从句时,如果主从句主语一致(或主从句主语不一致,但从句主语为it),从句谓语部分含有be,可将从句主语和动词be省略,构成省略式创新句型。

　　例1:I won't come to see you unless I am asked to. 除非你邀请我,我才会来看你。

　　析:因为主从句主语一致,从句谓语又含有am,所以可将从句主语I和动词am 省略,构成省略式创新句型:I won't come to see you unless asked to.

　　例2:The soldiers will not cross the river unless they are ordered to. 除非接到命令,战士们不会过河。

　　析:因为主从句主语一致,从句谓语又含有are,所以可将从句主语they和动词are省略,构成省略式创新句型:The soldiers will not cross the river unless ordered to.

　　注意:

　　until也可作为介词。例如:

　　I have remembered her until now. 直到现在我还记得她。

　　(文/郝昌明；英语辅导报高三版2003-2004学年第39期；版权归英语辅导报社所有，独家网络合作伙伴新浪教育，未经许可，不得以任何形式进行转载。)