结构
As mentioned above, unless is a conjunction which we use in conditional phrases. In written English, the clause that follows unless is the subordinate clause (SC) meaning that it needs a main clause (MC) to make a complete sentence. It is similar to how we use if in conditional phrases.
之前提过，unless是在条件句中使用的连词。在书面语种，使用unless的从句叫做从属句(SC)，需要主句(MC)让句子完整。这就像if在条件句里的用法一样。
When unless comes before a main clause we use a comma:
unless放在主句之前加逗号：
Unless it rains, we’ll go for a picnic tomorrow.
[en](SC) (MC)
[/en]
除非下雨，否则我们明天去野餐。
When the main clause comes first, no comma is required:
主句在前，不需要逗号：
They won’t come unless you invite them.
[en](MC) (SC)
[/en]
除非你邀请他们，否则他们不会来。
Unless is like If in that we don’t not use will/would after it. We only use the present simple tense.
像if一样，我们不会在unless后面使用will/would。只使用一般现在时。
Unless I hear from you, I’ll see you at 5pm.
除非你通知我，否则我和你下午5点碰面。
HOW DO WE USE “UNLESS”?
如何使用“unless”?
Unless and if not
unless和if not
Ok, so unless is similar to “if …..not” and together they mean “except if”. Note: We refer to real conditional situations and NOT impossible situations.
unless和if…not很相似，它们都表示“除非”。注意：我们指的是真实的条件情况，而不是不可能的情况。
If you don’t study, you will fail your exam./ Unless you study, you will fail.
如果你不学习，你考试就过不了。/除非你学习，否则你就过不了考试。
We could eat at Frankie and Benny’s if they’re not closed on a Monday/ We could eat at Frankie and Benny’s unless they are closed on a Monday.
如果弗兰基&班尼餐馆周一不关门，我们可以在那吃饭。/除非弗兰基&班尼餐馆周一关门，否则我们就在那吃饭。
I’ll make dinner if nobody wants to/ I’ll make dinner unless someone else wants to.
如果没人想做饭的话，我做吧。/除非没别人想做饭，那么我就来做吧。
If you don’t stop smoking, you will feel bad/ Unless you stop smoking, you will feel bad
如果你不戒烟，你会不舒服的。/除非你戒烟，不然你会感觉不舒服。
Note that the sentence after unless is always a positive sentence. You cannot have a negative sentence after unless because that would make the sentence a double negative and senseless.
注意：unless之后的句子总是肯定的。在unless之后不能接否定句，因为这就造成了双重否定，句子没有意义。
Example: Unless you don’t study, you will fail. (????)
例句：除非你不学习，否则你就会失败。(小编注：即使是此句的中文译文也是一个有语义问题的病句，双重否定后使句子无逻辑意义了，符合逻辑的句子应该是“除非你学习，否则你就会失败。”)
***Warning: Typical Error
警告：典型错误
We don’t use “unless” when we mean if.
表示“如果”的时候不用unless。
Example: If you feel ill, I can drive/ Unless you feel ill, I can drive
例句：如果你觉得不舒服，我来开车吧。/除非你觉得不舒服，否则我来开车吧。
Spoken English
口语
When speaking, we use unless to introduce an extra thought or piece of information:
说话的时候，我们用unless表达一些额外的思考或者是信息：
He didn’t even know about the crash – unless he’d heard about it on the radio.
他都不知道那场空难—除非他在广播上听说了。
A: Oh look. Neil next door’s got a new car.
A：看!尼尔的邻居有了辆新车!
B: Unless they’ve got a visitor.
B：除非他们来客人了。
HOW about “NOT UNLESS”?
not unless的用法呢?
Not unless is similar to only if
not unless和only if相似
Example:
例句：
Shall I tell Liz what happened? Not unless she asks you (= only if she asks you)
我需要告诉利兹发生的事吗?除非她问起来。
Will you come shopping with me? Not unless you offer me lunch (= only if you offer me lunch)
你和我一起去逛街吗?除非你给我提供午餐。
