英语题 if not 的用法

英语题 if not 的用法

1.当if…not…引导非真实条件分句时，一般不可改用unless。 例如：
　　If she had not had an alarm clock,she would have missedthe train.（事实：她有闹钟。）
　　如果把上述例句中的if…not…换成unless，意义就相反了。
　　（误）Unless she had had an alarm clock, she would havemissed the train.（事实：她没有闹钟）
　　2.If…not…引导的条件分句可以重叠使用，unless则不可， 因为unless表示唯一条件。例如：
　　She won't lose weight if she doesn't keep a diet and ifshe does not take exercise every day.
　　3.unless引导的分句可用否定结构，而if…not …引导的分句不可再否定。例如：
　　Don't ask me to explain unless you really don't understand.
　　The directors have a meeting every Friday,unless there is nothing to discuss.
　　以上两例句中的unless不可换成if…not…。