英语中值得注意的五类省略结构
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 favourite programme. 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。