unless if 在用法上的区别可以分为以下几点：

【1】当if…not…引导非真实条件分句时，一般不可改用unless。

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。

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。

Don't ask me to explain unless you really don't understand.

参考译文：别要求我给你解释，除非你真的不懂/明白/理解。

The directors have a meeting every Friday, unless there is nothing to discuss.

参考译文：这些主观/经理每个星期五都要开一次会议，除非无事讨论。

以上两例句中的unless不可换成if…not…。

【4】在unless引导的肯定形式的条件分句中， 通常用肯定词（即some, already, still, too, as well等），在if…not…引导的分句中，通常用非肯定词（即any, yet, ever, either, at all等）。

I will criticize you if you haven't spoken to her yet.

I will criticize you unless you have already spoken to her.

（误）…unless you have spoken to her yet.

不过在否定形式的非真实条件分句中，if…not …结构仍能与肯定词连用。

If he hadn't already prepared his lesson, he wouldn't beallowed to go.（因为这种从句表面上是否定，实际上是肯定。）

【5】在间接引语中，当if=whether，意为“是否”，unless 不可以代替if…not…。

She promised to let me know if she wasn't coming.

【6】当unless含有“除了”的意思，不可为if…not…所替代。例如：

He never stammers, unless when he is angry.( unless when=except when)

Nothing will come out of it, unless disaster.(unless=except)

【7】当主句是疑问句时，不可用unless代替if…not…。 因为unless用在A情况发生，必导致B结果之类的复合句中。

What shall we do it they don't reply to our letters?

【8】当主句谓语是be＋某些形容词或分词glad, surprised, disappointed, satisfied, ashamed, delighted和pleased等， 不可用unless代替if…not…。

I'll be really surprised if they don't come to the meeting.

I'll be quite glad if she doesn't join them.

之所以不能为unless代替，是强调否定条件的出现，表明在这种前题下的因果关系。以上条件状语从句都可变为原因状语从句。

I'll be really surprised that they won't come to the meeting.

这里if被that代替了，而从句的否定结构不变。