1. unless 有时可与 if…not互换，但此时通常只限于从句所引出的情况为主句情况的一种例外，即此时的unless意思是except if，如：

Unless you start at once you’ll be late.

=If you don’t start at once you’ll be late.

=Except if you start at once you’ll be late.

2. 当从句所引出的情况不是表示主句情况发生的一种例外而只是一种可能，并且这种可能正是导致主句情况发生的主要原因时，英语习惯只用if…not，不用unless，如：

I’ll be surprised if he doesn’t win.

She’d look nicer if she didn’t wear so much make-up.

I’ll be sorry if they do not come tomorrow.

3. 当从句内容与已知事实相反时（即引出虚拟条件时），通常要用if…not，而不用unless，如：

If he weren’t so silly, he would understand. (In fact he is silly.)

If I hadn’t stopped her, she would have jumped down. (In fact I stopped her.)

unless有时可引出一个过去未曾实现的假想法情况，对前面主句进行补充说明（此时必须置于主句之后，且前面常用破折号而不用逗号）。如：

I couldn’t have got to the meeting on time—unless I had caught an earlier train. (In fact I wasn’t there on time.)

此句若将 unless 改为 if…not，则成为：

I couldn’t have got there if I hadn’t caught an earlier train. (In fact I was there on time.)