if...not和unless通常是可以換用的.但在下列情況下,兩者是有區(qū)別的：
1.unless多引導(dǎo)真實條件句,if...not可以引導(dǎo)真實條件句或非真實條件句.
He won't be able to pass the final exams unless he works hard.
除非他努力,否則就通過不了期末考試.
He won't be able to pass the final exams if he doesn't work hard.
如果他不努力,就通過不了期末考試.
2.如果主句描述的是情感或情緒活動方面的面容,if...not結(jié)構(gòu)不能換成unless.
I'll be quite a glad if she doesn't come this evening.
她今晚如果不來我很高興.
3.unless引導(dǎo)的狀語人可用否定結(jié)構(gòu).
Don't ask me to explain again unless you really don't understand.
不要再叫我解釋了,除非你真的不懂.
4.unless能作為介詞使用,相當(dāng)于except.
Nothing will come out of it unless disaster.
這種事除了引起災(zāi)禍之外不會有什么結(jié)果.