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.
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。
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.