中学英语教材中出现了unless和if... not引导的条件状语从句。例如: ①As we know, heavy objects and light objects fall at the same speed unless air holds them back. ②If we hadn''t arrived at five, we could not have caught the train. 有些教师在讲解unless的用法时, 常常简单地把它与if... not等同起来。因而许多学生便得出一个结论: unless ＝if... not, 其实, unless与if... not不是在所有情况下都可以换用的。本文想谈谈二者在什么情况下可以换用, 什么情况下不能换用, 供大家参考。

　　一、如果if... not引导的是一个非真实条件状语从句, 而谓语动词又是be时, if... not一般不可改用unless。例如:

　　If I weren''t a teacher, I would not concern myself with your morality. 假如我不是老师的话, 我就不会关心你的道德品质了。（实际上我是老师, 因此就要关心你的道德品质。）

　　如果该句改用unless代替的话, 意思就相反了。

　　Unless I were a teacher, I would not concern myself with your morality. 我不是老师, 我不会关心你的道德品质。

　　但是, 当谓语动词是行为动词时, 则又可以换用了。下面两句的意思相同。

　　Unless she had noticed this, the world would have lost one of the greatest discoveries of the century. ＝If she hadnot noticed this, the world would have lost one of the greatestdiscoveries of the century. 假如她没有注意到这一点, 世界就会失去那个世纪一项最伟大的发现。

　　二、在表示“由于未发生B而发生A”的句子中, 只能用if... not, 而不用unless。例如:

　　I''d be glad if she doesn''t come this evening. 如果今天晚上她不来我会很高兴。

　　If the animals cannot find enough plants to eat, they will starve or have to leave the place. 要是那些动物找不到足够的植物吃, 它们就会饿死或被迫离开那地方。

　　三、在表示“如果B不阻碍A, A将发生”的句子中, if... not可以与unless换用。例如:

　　“If the policemen don''t find the wallet in my pocket”, the thief thought, “maybe they will let me go."＝“Unless the policemen find the wallet in my pocket,... 小偷想: “假如警察找不到我口袋里的钱包, 或许他们会放我走。”

　　I''ll telephone（to）your father if you aren''t here on time tomorrow. ＝... unless you are here on time tomorrow. 除非你明天按时来, 否则我就打电话告诉你爸爸。

　　四、unless表示“除……外唯一的条件”, 因此不出现and连接的两个unless从句 而if... not却可以, 这时if... not不能换成unless。例如:

　　You will not lose weight if you don''t keep a diet and if you don''t take exercise every day. 如果你不控制饮食, 如果你不每天锻炼, 你就不会减肥。

　　He won''t be pleased if he doesn''t study hard and if he doesn''t make great progress. 如果他不努力学习, 不取得很大进步, 他是不会高兴的。

　　五、unless引导的从句可以用否定, 而if... not引导的从句不可再加否定, 因此, 在unless引导的否定从句里, 不可用if... not代替unless。例如:

　　I will go unless no one else does. 没人去我就去。

　　不可说: I will go if no one else doesn''t.

　　Don''t ask me to tell the news unless you really don''t know it. 如果你不是真的不知道那消息, 不要叫我告诉你。

　　不可说: Don''t ask me to tell the news if you really don''t know it.

　　六、在if... not引导的从句中, 通常用非肯定词 而在unless引导的从句中, 通常用肯定词。试比较:

　　I will be very angry if you haven''t spoken to her yet. 假如你还没有告诉她, 我会生气的。

　　Iwillbe very angry unless you have already spoken to her.

　　但在否定形式的非真实条件从句中, 其实际意义是肯定的, 因而if... not从句可用肯定的。例如:

　　If he hadn''t already finished his homework, he wouldn''t be allowed to go. 他如果没有做完作业就不能走。（实际上他已经做完了作业。）