状语从句中的省略用法，如果从句的主语和主句的主语一致，且从句的谓语含有be动词的某种形式(am/is/are/was/were)，可同时省略从句的主语和be动词的某种形式。这是比较固定和常见的省略结构。
1.when, while引导的时间状语从句。
《剑桥雅思真题7》Test 3 Passage 1
Their fungus farming and aphid herding crafts are sophisticated when(they are)compared to the agricultural skills of humans five thousand years ago but have been totally overtaken by modern human agribusiness.
与五千年前人类的农业技能相比，它们的真菌养殖和蚜虫养殖工艺是复杂的，但已经完全被现代人类农业综合企业所取代。
《剑桥雅思真题7》Test 4 Passage 1
While(she is)perusing a book on the monuments of Egypt, she noticed a hieroglyph that showed a row of men standing in odd postures.
在翻阅一本关于埃及纪念碑的书时，她注意到一列象形文字，上面画着一排人以奇怪的姿势站着。
2.if, unless, once引导的条件状语从句。
《剑桥雅思真题7》Test 4 Passage 1
It offers many benefits and, if(it is)used well, can be an excellent tool for improving your own efficiency.
它提供了许多好处，如果使用得当，它可以成为提高效率的极好工具。
《剑桥雅思真题2》Test A Section 1
A university permit does not entitle them to park in Hall car parks however, unless(they are)authorized by the Warden of the Hall concerned.
不过，大学许可证并不赋予他们在大堂停车场停车的权利，除非得到有关大堂总监的授权。
《剑桥雅思真题5》Test 3 Passage 1
Once(they are)over the age of three, children continue to expand on existing knowledge of the world.
一旦超过三岁，孩子们就会继续扩展现有的世界知识。
3.though, although, whether, no matter whether/what/how/who,whatever, wherever, however等引导的让步状语从句。
《剑桥雅思真题7》Test B Section 2
If you decide to do this, tell the recipient a definite date when you'll be able to get back to him or her and try to keep to this wherever(it is)possible.
如果你决定这样做，告诉收信人一个明确的日期，你什么时候能给他/她回电话，并尽可能地保持这样做。
《剑桥雅思真题5》Test 2 Passage 1
Baekeland's invention, although(it was)treated with disdain in its early years, went on to enjoy an unparalleled popularity which lasted throughout the first half of the twentieth century.
贝克兰的发明，尽管在最初的几年里受到轻视，但后来获得了空前的普及，并持续了整个二十世纪上半叶。
《剑桥雅思真题9》Test 3 Listening Section 4
Wherever(it is)possible, recycled materials have been used.
只要有可能，回收材料就会被使用。
4.as if, as though引导的方式状语从句。
She talked to the stranger as if(she were)absent-minded.
她和陌生人谈话时似乎心不在焉。
She stood at the gate as though(she was)waiting for someone.
她站在门口好像在等人。
5.than和as引导的比较状语从句。
当不同的主语进行比较时，一般省略从句中的谓语;当从句中的主语与谓语(be动词除外)和主句中的主语与谓语相同时，通常省略从句中的主语和谓语，只保留比较部分。
《剑桥雅思真题7》Test 2 Passage 1
As(it is)in China, they were first introduced with Buddhism and were attached to important temples.
就像在中国一样，佛教首先传入中国，并依附于重要的寺庙。
《剑桥雅思真题7》Test 1 Passage 2
But such projects must be built to higher specifications and with more accountability to local people and their environment than(they are)in the past.
但这些项目必须按照更高的规格建造，并对当地人民及其环境承担比过去更多的责任。
