While the teacher was lecturing to the class, I fell asleep.
妙语点睛 这里从句的主语是the teacher，而主句的主语是I，两者不一致，因此状语从句不能简化成短语。此句若简化成现在分词短语说成：While lecturing to the class, I fell asleep.意思则是“当我在给这个班上课时，我睡着了”，这样显然不妥。
1 1) A zero can have its meaning only when it is used with real numbers; thoughts can give off brilliant light only when they are put into actions.
2) A zero can have its meaning only when used with real numbers; thoughts can give off brilliant light only when put into actions.
妙语点睛 这里两个when引导的从句中分别省去了it is和they are。这里的it指主句的主语zero，they指主句的主语thoughts。
2 1) A tiger can't be tamed unless it is caught very young.
2) A tiger can't be tamed unless caught very young.
妙语点睛 这里unless引导的从句中省略了it is。这里的it指主句的主语tiger。
3 1) Since I came to Beijing, I have made many new friends.
2) Since coming to Beijing, I have made many new friends.
4 1) After I finished my homework, I fed the dog.
2) After finishing my homework, I fed the dog.
5 1) After he jumped out of a boat, the man was bitten by a shark.
2) After jumping out of a boat, the man was bitten by a shark.
1 1) I turned off the TV in order that my roommate could study in peace and quiet.
2) I turned off the TV in order for my roommate to study in peace and quiet.
2 1) I spoke slowly and clearly so that/in order that the audience could understand me.
2) I spoke slowly and clearly in order for the audience to understand me.
3 1) They carved the words on the stone so that/in order that the future generation should remember what they had done.
2) They carved the words on the stone in order for the future generation to remember what they had done.
4 1) The teacher raised his voice in order that the students in the back could hear more clearly.
2) The teacher raised his voice in order for the students in the back to hear more clearly.
1. While I was waiting for the bus, a brick fell on my head.
2. After I turned to the corner, I saw a tile fall off the roof.
3. After I turned to the corner, a tile fell off the roof.
4. When I opened the door of the refrigerator, the smell was bad.
5. While I was watching TV last night, the telephone rang.
6. Before I came to class, I had a cup of coffee.
7. After he had finished breakfast, he left the house and went to his office.
8. After she had completed her shopping, she went home.
9. Alex hurt his back while he was chopping wood.
10. You should always read a contract before you sign your name.
11. While I was trying to get to sleep last night, a mosquito kept buzzing in my ear.
12. Since we arrived here, we have made many new friends.
13. After we looked at the map, we tried to find the right street.
1. Oceans continually lose by evaporation much of the river water .
A. to constantly flow into them
B. is constantly flowing into them
C. constantly flows into them
D. constantly flowing into them
2. Tornadoes, powerful, destructive wind storms, occur most often in the spring when hot winds over flat land encounter heavy, cold air.
A. which to rise
B. that rising
C. are rising
3. In 1938 Pearl S. Buck became the first American woman the Nobel Prize for Literature.
C. to receive
D. she received
4. The first explorer California by land was Strong Smith, a trapper who crossed the southwestern deserts of the United States in 1825.
A. that he reached
B. to reach
D. reaching it
5. often found in fruit and vegetables.
A. Vitamin C, a trace element that is
B. For vitamin C, a trace element to be
C. Vitamin C, a trace element, is
D. Vitamin C, is that trace element.
6. The Amazon rain forests, the earth's lungs, convert carbon dioxide in the atmosphere back into oxygen.
A. functioning as
B. which functioning as
C. functions as
D. functioned as
7. Through a process coalescence, water droplets in clouds grow to a size large enough to fall to earth.
B. to be called
8. Penicillin, in the early 20th century, brought in the golden age of chemotherapy.
A. to be discovered
C. discovery was
9. The mouse, like the keyboard, is a control device to a computer.
B. to connect it
C. and connect
D. that connect
10. An amendment to the Constitution in Harry Truman's tenure limits the US presidency to two terms.
B. to pass
D. was passed
11. A reagent is any chemical that reacts in a predictable way with other chemicals.
A. when mixed
B. when is mixed
C. it mixed
D. when mixing
12. A beam of light will not bend round corners unless to do so with the help of a reflecting device.
B. to be made
C. being made
D. having made
13. As a general rule, snakes don't bite unless .
B. are offended
C. they offended
14. She had said little so far, responding only briefly when .
C. speaking to
D. spoken to
15. , his family moved to America.
A. When still a baby
B. When a baby still
C. When he was still a baby
D. When being still a baby
16. native to Europe, the daisy has now spread throughout most of North America.
B. If it were
C. In spite of
D. That it is
17. In its pure state antimony has no important uses, but with other substances, it is an extremely useful metal.
A. when combined physically or chemically
B. combined when physically or chemically
C. the physical and chemical combination
D. it is combined physically and chemically
18. Although pure diamond is colorless and transparent, with other material it may appear in various colors, ranging from pastels to opaque black.
A. but when contaminated
B. but when contaminating
C. when contaminated
D. when contaminating
19. classified as a carnivore, the North American grizzly bear eats berries and even grass.
A. Just as
B. Because of
20. usually thought to end in northern New Mexico, the Rocky Mountains really extend southward to the frontier of Mexico.
B. To be
21. Nearly all trees contain a mix of polymers that can burn like petroleum properly extracted.
C. when it
22. Although rigid, bones exhibit a degree of elasticity that enables the skeleton to withstand considerable impact.
B. are apparently
C. apparently their
D. are they apparently
23. Although of the world's oceans, the Atlantic has by far the largest drainage area.
A. the largest is not
B. not the largest
C. the largest not
D. largest not the