I．要点
　有时为了避免重复，使语言简练紧凑，在不损害结构或引起误解的原则下，往往省去一个或多个句子成分或词语。
　１、固定习惯用词。如：
　No smoking! Thanks s lot!等。
　2. 简单句中的省略
　（１）口语中，一、二、三人称的主语，有时还包括谓语都可以省略。如：（It is）Nice to see you!
　(This is) Li Ming speaking.
　（２）所有格后的名词如为住宅、商店、工矿、教堂等可以省略。如：
　I'm going to visit Tom's (house).
　I met him at the tailor's (shop).

　（３）两个或两个以上的动词不定式并列在一起时，第一个不定式带to，后面的不定式可省to.如: His job is to clean and mend the machine.

　（４）主（宾）语补足语中的to be常省略。He was considered (to be) the best student in the class.
　（５）There be结构中there be可同时省略，或只省略there.如：(Is there) Anything wrong?
　（６）表示年龄的years old, 表示钟点的o'clock, minute等常省略。如：What time is it now? It's ten (o'clock).
　3从句中的省略
　（１）宾语从句，以which, when, where, how和why引起的宾语从句谓语与主句谓语如相同，可省略从句中全部谓语，甚至主语也可省略，仅保留wh－一词。如：
　He will come, but we don't know when (he will come).
　He didn't come, I wondered why (he didn't come).
　（２）定语从句中可省略作宾语的关系代词，如：
　The man (whom) I saw in the street the other day is my teacher.
　（３）状语从句，在时间、地点、让步、方式、条件状语从句中，从句主语与主句主语一致，或从句主语是it，则be动词及其主语常可省略。如：
　I'll tell him that when (it is) possible.
　I won't go there unless (I'm) invited.

II．例题
　例１Abeam of light will not bend（弯曲）round corners unless____ to do so with the help of a reflecting device（反射装置）。
　A made B being made C having made D to be made
　解析：该题正确答案为A。unless后省略了it is. make sb (sth) do sth变成被动语态则为sth/ sb be made to do.
　例２While ____my homework, I heard a cry for help.
　A do B did C doing D having done
　解析：该题答案为C。该空处省略了I'm,相当于while I'm doing my home work.如：While playing guitar, he is singing.