1.unless在引导条件状语从句时，与If……not / Except if意思相近。unless引导条件状语从句含义是“如果不”时，跟if引导条件状语时相同，遵循主将从现原则，即主句用将来时，unless从句用一般现在时表将来。因此unless从句中，不会出现will/would。如：
We’ll have to cancel the show unless we sell more tickets at the last minute.
We’ll have to cancel the show except if we sell more tickets at the last minute.
We’ll have to cancel the show if we do not sell more tickets.
如果我们在最后时刻没卖出更多的票，就不得不取消演出。
You will feel cold unless you wear a coat.
You will feel cold if you don't wear a coat.
如果你不穿上外套，就会觉得冷。
Unless it rains, we’ll go for a picnic by the river tomorrow.
We’ll go for a picnic by the river tomorrow if it doesn’t rain.
如果不下雨，我们明天将去河边野餐。
Unless you work hard, you will not pass the exam.
If you do not work hard, you will not pass the exam.
如果你不努力学习，就不能通过考试。
I'll arrive at 10am unless there is traffic.
I'll arrive at 10am if there isn't traffic.
如果不堵车，我上午10点就能到。
改写:
You must finish the job in time. Otherwise, you will be sacked.
If you do not finish the job in time, you will be sacked.
Unless you finish the job in time, you will be sacked.
如果你不及时完成工作，就会被解雇。或：
除非你及时完成工作，否则你将被解雇。
You must not violate the rules. Otherwise, you will be in trouble.
If you violate the rules, you will be in trouble.
Unless you violate the rules, you will not be in trouble.
如果你不违反规定，你就不会有麻烦。或：
除非你违反规则，否则你不会有麻烦。
The boys must reach school in time. Otherwise, they will be punished.
If the boys do not reach school in time, they will be punished.
Unless the boys reach school in time, they will be punished.
如果男孩子们不及时到达学校，就会受到惩罚。或：
除非男孩子们及时到达学校，否则将受到惩罚。
unless从句需是肯定句。
The government cannot operate effectively unless it is free from such interference.
除非政府能排除这种干扰，否则将无法有效运转。
The gym is for members only. You can’t go unless you are a member. =except if
健身房只对会员开放。除非你是会员，否则你进不去。
I’ll see you this afternoon unless I have to work. =except if
除非我得工作，否则今天下午见。
We can walk to the restaurant, unless you would prefer to drive. =except if
我们可以步行去餐厅，除非你更愿意开车去。
We cannot understand disease unless we understand the person who has the disease.
除非我们了解患此疾病的人，否则我们无法了解此疾病。
unless can’t be used with a question.
unless不能用于疑问句中。
What will you do if you do not pass this final exam?
如果没有通过这次期末考试，你会怎么做？
2.We don’t use unless for impossible conditions, we use if.
unless一般不用于非真实条件句。if既可用于真实条件句，也可用于非真实条件句。
If you hadn’t studied hard, you’d have failed the exma.
假如你没有努力学习的话，你就不会通过考试了。
She’d be better caopany if she didn’t complain so much,
如果她不是喋喋不休地抱怨，那她就是一个较好的伴侣了。
If the government had not raised food prices, there would not have been so many protests.
要是政府没有提高食品价格，就不会有这么多的抗议活动。
3.In spoken English, we use UNLESS to introduce an extra thought or piece of information.
在英语口语中，主句在陈述了一个事实后，可用unless来引出一个额外（例外）的想法或信息、情况、特殊。这个时候也不能用：if……not改写句子。
We had no snow this winter, unlessin the mountain districts.
今年冬天没有下雪，但山区除外。
He didn’t even know about the crash, unless he’s heard about it on the radio.
他甚至不知道这起车祸，除非他从广播中听到了。
Look! Andy got a new car in front of his house – unless he’s got a visitor.
快看！安迪家门口停了一辆新车 – 除非他家有访客。
He arrived at the inn late at night and asked the handlord for a room,
“There’s nothing left,”he said,“unless you’d like to sleep in Number 7.”
他深夜到达旅馆，向老板要了一个房间。
“什么都没有了，”他说，“除非你想睡在7号。”
I couldn’t have got to the meting on time—unless I had caught an earlier train.
句子含义：我没有准时到会，随后引出一个想法：只有在赶上更早的火车的情况下才可能准时到会。
译为：那时我是没有可能准时到会的——除非我赶上了较早一班的火车。
如果硬要用if……not改写句子，那么句子含义发来变化：
I couldn’t have got to the meting on time if I hadn’t caught an earlier train.
要是我没有赶上较早一班的火车的话，我就不能准时到会了。（虚拟条件句中的非真实条件句），再比较：
I couldn’t have got to slough in time unless I’d had a helicopter.
我那时是绝没有可能及时赶到斯劳市的，除非当时我有一架直升机。
I couldn’t have got to slough in time if I’d not had a helicopter.
如果那时没有一架直升机的话，那我是不可能及时赶到斯劳市的。
4.当unless引导出分词短语时，也不能用if……not改写句子：
Unless otherwise instructed, you should leave by the back exit.
如果没有额外的通知，你必须从后面的出口出去。
Unless paying by credit card, please pay in cash.
如不使用信用卡，请付现金。
Unless changed, this law will make difficult for farmers.
这项法律如不修改必将给农民的生活带领困难。
5.not unless = only if 含义是只有在……条件下才
I won't tell them ─ not unless you say I can.
我绝不告诉他们——除非你允许。=
I won't tell them ─only if you say I can.
“Shall I tell Andy what happened?” “Not unless he asks you.”
我应该告诉安迪所发生的事吗？除非他问你，否则不用。=
“Shall I tell Andy what happened?” “Only if he asks you.”
Will you go with her?” “Not unless she wants me to.”
你会和她一起去吗？除非她要我去，否则我不去。=
Will you go with her?” “Only if she wants me to.”
“Will I have to pay?” “Not unless you want to.”
我必须付钱吗？除非你想，否则不用。=
“Will I have to pay?” “Only if you want to.”