if...not和unless通常是可以换用的.但在下列情况下,两者是有区别的：
1.unless多引导真实条件句,if...not可以引导真实条件句或非真实条件句.
He won't be able to pass the final exams unless he works hard.
除非他努力,否则就通过不了期末考试.
He won't be able to pass the final exams if he doesn't work hard.
如果他不努力,就通过不了期末考试.
2.如果主句描述的是情感或情绪活动方面的面容,if...not结构不能换成unless.
I'll be quite a glad if she doesn't come this evening.
她今晚如果不来我很高兴.
3.unless引导的状语人可用否定结构.
Don't ask me to explain again unless you really don't understand.
不要再叫我解释了,除非你真的不懂.
4.unless能作为介词使用,相当于except.
Nothing will come out of it unless disaster.
这种事除了引起灾祸之外不会有什么结果.