在英语教学中，我们发现学生在使用貌似简单的unless时经常出错，众多词典和语法书讲解又不够详细，因此本文拟就以下方面作必要说明。

   
    一、unless的基本用法
    连词unless意为“除非……如果不……”，“除了……”，多引导一个否定意义的真实条件句，有时也可引导非真实条件旬。unless引导条件句时，主要用于下列情况。
    (1)主句为肯定句。如：
    Unless I visit every bookstore in town．I shall not know whether I can get what l want．如果我不到城里每一家书店去看看，我就不知道是否能买到我想要的书。
    You will miss the bus unless you hurry up．你要不快点就会错过班车。
    You’ll fail in chemistry again unless you work harder．如果你不再加把劲，你化学考试还会不及格。
   

(2)主句为否定句。如：
    Unless you oil the motor regularly．it won’t run smoothly．要不是你经常给发动机上油。它就不会转得那么顺畅。
    I wouldn't be saying this unless I were sure of the facts．要是我对这些事情没有把握，我就不说这话了。
    You will never get anywhere unless you have set your goal．如果你不设定目标，你哪儿也去不了。
   

二、应注意的一些问题
    1．unless不能与if'"not换用的情况
    unless在意义上相当于if...not，因此在有些场合中unless与if...not可互换使用。
如：
    Unless I am mistaken．I’ve seen that man before．=lf I am not mistaken。I’ve seen that man before．如果我没搞错，我以前见过那个人。
    Unless the strike has been called off，there will be no trains tomorrow．=lf the strike has not been called off。there will be no trains tomorrow．除非罢工结束了，明天才会有火车。
    但在有些情况下，二者不可互换使用。如：
    (1)当unless引导非真实条件句时，一般不可改用if...not。如：
    They couldn’t afford to live in such an expensive apartment unless they were rich．除非他们很富有，否则就住不起这么昂贵的公寓。
    They couldn’t have arrived at the site instantly unless the police had had a helicopter．除非警方有直升机，不然他们就不会即刻赶到事发地点。
    以上两例中，unless从句均表示与事实相反的情况。如果把unless换成if...not(…if they were not rick,…if the police had not had a helicopter)就与原句意义完全相反了。
    (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，不能用and连接两个unless引导的从句。
如：
    She won’t lose weight if she does not keep a diet and if she does not take exercise every day．=She won’t lose weight unless she keeps a diet and takes exercise every day．除非她坚持节食、每天锻炼，否则是不能减肥的。
    (3)unless引导的条件句可用否定结构，而if...not引导的条件句不可再加以否定，因为两个not不能重叠使用，因此在unless引导的否定条件句中，不能用if...not替代unless。如：
  Don't ask me to explain unless you really don’t understand．除非你真不懂，不然别让我解释。
    The directors have a meeting every Friday．unless there is nothing to discuss．董事会每周五开会，无事讨论除外。
    You will get a good grade in the course unless you don’t do your homework．如果你(不是不)做作业的话，你就会有好的学业成绩。
    (4)当if引导名词性从句时，unless不可以代替if...not，因为if=whether(“是否”)。如：
    It is unimportant if he will not attend the graduation ceremony．他是否参加毕业典礼无关紧要。
    I doubts if he has not made a contribution to the environment．我怀疑他是不是没有为环保做过贡献。
    (5)当unless作介词(“除……以外”，“除非”)时，if...not不能与其互换使用。如：
    Nothing will come out of it．unless(=except)disaster．这件事除了引起灾祸之外，
不会有什么结果。
    Nothing．unless a miracle，can save him．除非出现奇迹，否则他无法得救。
    2．unless从句中成分的省略
    unless从句的主语与主句的主语相同时，unless从句中的主语和谓语(或一部分)有时
可省略。如：
    Unless(he is)in uniform。he doesn’t look like a policeman．如果不穿上制服，他看起来就不像警察。
    Unless(I am)compelled t0 stay in by bad weather，I go for a walk every day．若非天气不好被迫呆在家里，我每天都出去散步。
    Don't leave the building unless(you are)instructed to do so．没有得到通知，不要离开大楼。
    The virus is capable of destroying an information stored in a computer unless(it is)detected and eradicated．病毒如不查出并清楚掉，会把存储在计算机里的资料全部毁掉。
    3．unless从句的时态
    主句是一般将来时态、祈使句或谓语中含有情态动词或主句是一般现在时，unless引导的真实条件句用一般现在时；如果强调从句动作发生在主句动作之前，从旬可用现在完成时。
    Don't touch the instruments unless the teacher allows you．没有老师的允许，不要乱动仪器。
    Unless you know a word，you may consult a dictionary．如果你不认识哪个词，你可以查词典。
    但主句是过去将来时或过去时，unless引导的真实条件句常用一般过去时。如：
    The disease prevented him from remembering things unless he was reminded by another．这种病使他记不住事，只能靠别人提醒。
    4．“祈使句(肯定式)+or／otherwise+陈述句”与“unless条件句+主句”间的转换
    “祈使旬(肯定式)+or(else)／otherwise+陈述句”与“unless条件句+主句”间有时可以相互转换。如：
    Put on your raincoat or you’ll get soaked．=Unless you put On your raincoat，
you’ll get soaked．If you don’t put on your raincoat，you’ll get soaked．把雨衣穿上，否则你就会全身湿透的。
    Store the meat in the fridge，otherwise it will go the fridge，otherwise it will go off．If you don't store the meat in the fridge，it will go off．把肉放在冰箱里，不然就会坏掉。
    5．在unles引导的肯定形式的条件句中。通常用肯定词(some,already, still,too,as well等)；在if...not引导的条件句中．通常用非肯定词(any．yet．ever．either．at all等)
    You ll never really know what happiness is unless you have something to compare it to．只有通过对比，才能了解幸福的含义。
    1 will inform her if you haven’t spoken to her yet．如果你还没跟她说，我通知她。
    不过在if…not引导的非真实条件句中，仍能用肯定词，因为这种条件句形式上是否定，意义上却是肯定。如：
    If he hadn’t already prepared his lesson，he wouldn’t be allowed to go．要是他没有做完功课，是不会允许出去的。(实际已做完功课)
    6．unless可用来引导出补充前言的句子
    unless引导出补充前言的句子，即对刚说过的话又突然想到要补充的内容。如：
  He hasn’t got any hobbies  unless you call writing a hobby．他没有任何嗜好——除非把写作也算作嗜好。
  Let’s play chess  unless you really have something urgent to do．下盘棋吧——除非你真有急事要办。
    最后要指出的是，在英语中，unless使用频度较高，极易在写作或翻译上产生错误，因此我们在教学中应要求学生多查资料、多思考、多练习，指导他们正确使用unless。



注意 unless 与 if … not 的区别：虽然 unless 与 if … not 的意思一样，但 unless 只能用于真实条件句，而 if … not 除了可用于真实条件句之外，还可用于非真实条件句。例如：

Unless you try, you will never succeed. = If you don't try, you will never succeed.

If he were not present now, I would go away. （非真实条件句）


if not表示不然的话，否则。后面不接句子，应该接短语，如If not his help, I would have failed.
Unless表示除非，如：
My baby sister never cries unless she is hungry.
我刚出生的妹妹除非饿了，她是从来不哭的。
I shall go there tomorrow unless I'm too busy.
如果我不太忙，明天将到那儿去。
Unless you go at once you will be late.
如果你不马上走，就会迟到的。

I couldn't have got to Slough in time unless I'd had a helicopter. 英
　　除非我有一架直升飞机, 否则我是不可能在那个时候赶到S地的.
　　 一般的英语语法书和辞典都会谈到, 连接词unless的涵义和if...not相同,它们可以互换使用. 例如: I shan't go unless he asks me. 等于I shan't go if he does not ask me. (Eckersley's A comprehensive English Grammar); You will fail unless you work harder. 等于You will fail if you do not work harder. (A. S. Hornby's The Advanced Learner's Dictionary of Current English).

　　其实, 这样的提法是不够全面的, 因为存在着这么一个问题: unless和 if...not是否在各种场合下都能互换使用呢? 现以上例--I couldn't have got to Slough in time unless I'd had a helicopter. 为例来探讨一下这个问题.
　　 此句引自国外报刊. 说话人被控犯了谋杀罪, 在法庭他自辩说:"发生此案时,我并不在S地, 而是在离S地几十里外的另一个小镇上, 关于这点我可以提供人证......". 接着他说道:"I couldn't have got to Slough in time unless I'd had a helicopter."
如果遵照一般语法书所述的公式: unless=if...not, 将此句改作I couldn't have got to Slough in time if I'd not had a helicopter, 那会产生怎样的后果呢? 若改作if...not, 其涵义是: I did have a helicopter and I did get to Slough in time to commit the murder.
　　 显而易见, 这并不是他所想说的话. 国此, unless是在大多数场合下可与if...not互换使用, 但在某些场合里(例如在某些谓语动词用虚拟语气的句子里)却不能相互置换,否则它们会产生截然相反的涵义．

　　 值得注意的是, unless用在虚拟语气句中时, 存在着两种可能性. 在某些句子中, 它表示if...not的涵义, 可以和if...not互换使用. 例如:
　　 Unless you had lent me a hand, I should not have fulfilled my task．
　　 = If you had not lent me a hand, I should not have fulfilled my task.
　　 要不是你们来帮忙, 我是不可能完成任务的.

　　 然而, 在另一些场合里, unless并不是表示 if...not的涵义, 而是表示近似于but...if的涵义. 这时候, 可以改写为but...if, 但决不能用if...not去替代. 例如, 他在法庭上所说的那一句, 可以改写为: I could not have got to Slough, but I could have got to Slough if I had had a helicopter.
　　 还有一点应当注意的是, unless句型和if...not句型具有没的倏色彩和表达风格, 前者是积极的说法, 态度明朗, 语气有力; 后者是退一步的说法, 态度含糊, 语气软弱无力.

　　 现在, 你能否说出下列两个句子的不同涵义?

　　 I should not expect their help unless they were my friends.
　　 I Should not expect their help if they were not my friends.

英语相关词语辨异词典

if：unless—448

1a．这两个连词虽非同义——前者是作“如果”讲（参见词目446、447、449和450），后者是作“除非”讲，相当于if not 的用法，有时却容易混淆。

比较：If the water is pure，it will not need further treatment．

如果水是清澈的，就用不着再加以处理了。

Unless the water is pure，it will need further treatment．除非水是清澈的，否则就需要再加以处理了。

You can see him if it's urgent．要是情况紧急的话，你可以去见他。

You can't see him unless it's urgent．除非情况紧急，否则你不能去见他。

1b．因此，前者要连接否定的条件从句，后者则要连接肯定的条件从句。

比较：If you don't study hard，you will not pass the examina- tion．你要是不用功读书，你就考不上。

Unless you study hard，you will not pass the examination．除非你用功读书，否则你就考不上。

If he has no objection，we will carry out the plan．他要是不反对，我们就照计划进行吧。

Unless he has some objection，we will carry out the plan．除非他反对，否则我们就照计划进行。

1c．同前者不一样，后者不宜用于虚拟结构。（注：只在文学作品中我们常能见到把它用于虚拟结构。）

例如：If he weren't so silly he would understand what you have said．要是他不那么笨拙，他准会理解你所说的话的意思。

（不说Unless he were so silly…）

If I hadn't stopped her she would have jumped．要是我没有阻拦她，她准会跳楼。（不说Unless I had stopped her…）

1d．同后者又不一样，由前者连接的从句往往含有“捉摸不定”的义蕴，所以可以加饰ever这个副词和any这个形容词。

例如：If you ever have any trouble，just let me know．你如有什么困难，就通知我好了。（不说If you ever have some trouble…）

I won't phone you，unless something unforeseen happens．要是没有什么意外之事发生，我就不打电话给你了。（不说…，unless anything unforeseen happens）

1e．唯一相同的是：两者都可以构成省略结构——省去主语和用作系动词的be。

例如：If（you are）in need，please don't hesitate to let us know．

如有需要，请你尽管通知我们好了。（可以省去“you are”） The baby is never peevish，unless（it is）sick．小宝宝不生病，决不会吵吵闹闹的。（可以省去“it is”）

quotes with unless:
A woman should never be seen eating or drinking, unless it be lobster salad and Champagne, the only true feminine and becoming viands.                Lord Byron

I don't feel right unless I have a sport to play or at least a way to work up a sweat.    Hank Aaron

No great poetry, of whatever kind, is conceivable unless the subject has become integrated with the poet's mind and mood.        Lascelles Abercrombie

I'm not going to just say nice things about everybody unless I mean it.    F. Murray Abraham

Unless you're willing to have a go, fail miserably, and have another go, success won't happen.   Phillip Adams

1.  If I _____  you, I ____  very happy.

A. were, would be     B. am, shall be

C. were, shall be      D. am, would be

2.  If he had not lost his money, he ____ the piano last week.

      A. would  buy        B. had  bought

      C. could have bought    D. bought
3. ____I known it, I should have told him.

       A. Have    B. Had   C. Having   D. If
4. ____ the clouds, you would find the airplane in the sky easily.

    A. Were it not for    B. If it had not been for

     C. If it were not      D. Had it not been for

5. If I had known that, I ___  so.

       A. wouldn’t do     B. won’t do

       C. didn’t do      D. wouldn’t have done

6. If it ___ so hard, we’d go to town.

     A. isn’t raining   B. weren’t raining

     C. doesn’t rain     D. isn’t going to rain
1.答案A    2答案C  3.答案B  4.答案A  5.答案D  6.答案B
7. He _____ you if you ____ to see him that afternoon.

  A. might tell, were going   

  B. told, were going

  C. might have told, had gone

  D. should tell, went

8. If you had started sooner, you ____ there.

      A. may have got      B. might have got

      C. shall have got      D. must have got

9. If we were to be 2oo years old, we ____ everything.

A. will change      B. can change

C. could change     D. shall change 

10. I don’t think that I shall fail, but if I ___ I would try again.

A. fail             B. would fail

C. should fail        D. had failed

11. If you ____ to see Mary, what would you tell her?

A. are              B. will be going
C. must             D. were

12. If the train leaves at half past ten, there ____ no time to lose.
A. would            B. is　　C. will              D. has
7.答案C   8.答案B   9.答案C   10.答案C  11.答案D　12.答案B

unless…“除非，如果不……”(=if…not)

[例句] wI shall go tomorrow unless it rains. 如果不下雨，我明天去。

–Shall Tom go and play football?

–Not unless he has finished his homework. 除非他完成作业，否则不能出去。

I won’t go unless he comes to invite me himself.w除非他本人来邀请我，不然我是不会去的。

I won’t attend his birthday party unless invited (=unless I am invited).  除非被邀请，否则我不

去参加的生日晚会。

1. The men will have to wait all day ______ the doctor works faster.（2001北京春季）

  A. if      B. unless   C. whether    D. that       

2. You will succeed in the end _____ you give up halfway.（2001上海）

  A. even if  B. as though  C. as long as  D. unless     

3. ______ you call me to say you’re not coming. I’ll see you at the theater.（2004吉林）

  A. Though  B. Whether  C. Until   D. Unless