一、由if构成的省略
这类省略结构最重要的有 if so, if not, if you must三个。如：
Is the book available, and if so where? 有没有这本书？如果有的话，在哪里？
Are you a student with a knack for coming up with great ideas? If so, we would love to hear from you. 你是那种能想出好主意的学生吗？如果是，我们倒想听一听。
I hope to see you tomorrow, but if not, leave me a message. 我希望明天跟你见面，不过如果不行的话，请给我留个口信。
I think I can fix it tomorrow. If not, you’ll have to wait till Friday. 我想我明天就可以修好它，如果不行，你就只好等到星期五了。
The snow was now two feet deep, making it difficult, if not impossible, to get the car out. 雪有两英尺厚，要把汽车拉出来很困难，尽管不是不可能。
“I’m just going to finish this off first.” “Go ahead then, if you must.” “我只是打算先把这件事做完。”“如果你非得要做的话，那就继续吧。”
二、由从属连词后接形容词构成的省略结构
这类省略比较重要的是由从属连词if, when, where, whenever, wherever等，后接形容词necessary, possible等构成。如：
Taste the soup and add salt and pepper if necessary. 尝尝这汤，需要的话加一点点盐和胡椒粉。
It is always best to choose organically grown foods if possible. 如果可能，最好还是挑选有机食品。
They are not afraid to tackle the issues or let the sparks fly when necessary. 他们不怕应对这些问题，也不怕必要时来一番激烈的争论。
I recommend that you avoid processed foods whenever possible. 我建议你尽量不要食用加工好的食品。
Changes can easily be made where necessary. 在需要的地方可以很容易地进行修改。
They believed in the application of force wherever necessary. 凡属必要的地方他们都主张使用武力。
Researchers try to quote primary sources wherever possible. 研究人员尽可能地引用原始资料。
三、由从属连词后接现在分词构成的省略结构
这类省略比较重要的是由从属连词as if, as though, once, when, while 等后接现在分词构成，注意此时现在分词的逻辑主语通常应与句子主语一致。如：
He paused as if expecting Lanny to speak. 他停了停，仿佛等待兰尼说话。
He rubbed his eyes and yawned as though waking up after a long sleep. 他又揉眼睛又打哈欠，好像睡了一大觉刚醒似的。
On the other hand, once having seen this apartment, who’d have the courage to complain? 另一方面，一旦看过这套公寓房，谁还会有勇气发牢骚呢？
They were surprised by her openness when talking about her private life. 她谈起私生活时非常坦率，大家都很吃惊。
He had jumped bail last year while being tried on drug charges. 去年他在因涉毒被控告期间弃保潜逃。
If the TV was built into the ceiling, you could lie there while watching your favorite program. 如果把电视嵌入天花板，你就可以躺在那里欣赏你最喜欢的节目了。
四、由从属连词后接过去分词构成的省略结构
这类省略比较重要的是由从属连词although, as, as if, as though, if, once, unless, until, when, while 等后接过去分词构成，注意此时过去分词的逻辑主语通常应与句子主语一致。如：
Although never married, they lived together as husband and wife for fifty years. 虽然从未结婚，但他们像夫妻一样在一起生活了50多年。
I am sending you information on holidays as promised. 正如所许诺的那样，我现在给你寄去关于度假的资料。
He was speeding down the motorway as if pursued by a demon. 他在公路上高速行驶，就像有魔鬼在追赶他似的。
If accepted for this post, you will be informed within a week. 如果你被接受担任这个职务，将在一礼拜内给你通知。
Once exposed to light, the film will be damaged and get useless. 一旦曝光，胶卷就要被损坏而不能用了。
The school rules state that no child shall be allowed out of the school during the day, unless accompanied by an adult. 校规规定，除非有成年人的陪同，孩子在白天不允许出学校。
Grill for 15 to 20 minutes, turning occasionally until cooked. 烧烤15分钟至20分钟，不时翻转直到烧熟为止。
When asked for your views about your current job, on no account must you be negative about it. 被问及你对自己目前工作的看法时，无论如何不要流露出消极情绪。
五、省略后只由wh-词来代替整个从句
有些由when, where, who, which, how等引导的从句，如果上文的语境清楚，可以从句省略，而只保留wh-词。如：
I’d like to help in some way, but I’m not sure how. 我想帮点忙，但不知道该怎么做。
Be very clear about what jobs should be completed, and by when. 对哪些工作应该要完成和应该什么时候完成必须非常明确。
I’ll call you ahead of time to fix up exactly when and where. 我会提前打电话给你，确定时间和地点。
I’ve left my keys somewhere and I don’t know where. 我把钥匙放在某个地方了，但不知道是哪个地方。
Miranda was sure it was one of them, but was not sure which. 米兰达确定是其中的一个，但不敢肯定是哪一个。
It’s either Spanish or Portuguese that she speaks, but I’ve forgotten which. 她说的要么是西班牙语，要么是葡萄牙语，但我忘记是哪一种了。
“Who told you?” “Oh, somebody or other, I’ve forgotten who.” “谁告诉你的?”“啊，有个人，我忘了是谁了。”
He’s angry with me and I don’t know why. 他生我的气了，但我不知道是为什么。
She gets periodic headaches, but she doesn’t know why. 她时不时地感到头疼，但不知道为什么。
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