英语问题,如何区别not...unless...,not...until...和not...before...

until he has come 强调直到.才 是一种临界状态
直到他回来我才完成作业
before he came 强调再他回来之前 我没法完成作业
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.

从意思上

1.The rain started at eight a.m. and did not stop until eight p.m.
意为：雨早上八点开始下，晚上八点才停。
2.The rain started at eight a.m. and did not stop before eight p.m.
意为：雨从早上八点一直下到晚上八点。
not...unti...