一、有形式主语it的主语从句可省略that。
1. It seems (或appears) (that) Joe is out，not Jack.
外出不在的似乎是乔，不是杰克。
2. It is an honor (that) I was invited to your birthday party.
我很荣幸被邀请参加你的生日聚会。
3. Its a pity (that) you cant operate a computer.
很遗憾，你不会操作计算机。
4. It is the third time (that) I have come to China.
这是我第三次来中国。
二、在when，while，if，as if，though(或although)，as，until，once，wheth er，unless，where等连词连接的状语从句中，常省略跟主句相同的主语和be动词。
1. When (water is) pure，water is a colorless liquid.
水纯净时，是无色的液体。
2. When (I am) in trouble I always turn to her for help.
我困难时总是找她帮助。
3. Errors，if any，should be corrected.
如果有什么错误，就应当改正。(if后省略了there are)
4. Wood gives much smoke while (wood is) burning.
木头燃烧时，它放出很多烟。
5. The letter is to be left here until (it is) called for.
这封信留在这里待领。
6. Henry looked about as if (he were) in search of something.
亨利向四周环视，似乎在寻找什么。
7. She studies very hard though (she is) still rather weak.
她尽管体弱，但学习仍十分努力。
三、当见到“when (或if，where，wherever，whenever，as soon as，as fast a s，than等)+possible/necessary等”时，可理解中间省略了it is(或was)。
1. Answer these questions，if (it is) possible without referring to the book.
如果有可能，请不看书回答这些问题。
2. When (it is) necessary you can help us to do something.
必要时你可以帮助我们做些事。
四、并列复合句中某些相同成分的省略。
1. This beeper works well，but that one doesnt (work well).
这个寻呼机工作正常，但那个就不行。
2. All uranium atoms do not have the same atomic weight. Some of them weigh 234 units，some (of them) (weigh) 235 units，and some (of them) (weigh) 238 units.所有的铀原子并非都有相同的原子量。其中有的重234，有的重235，而有的重238。
五、在限制性定语从句中可省略作宾语的关系代词whom，which，that。
1. That is the naughty boy (whom/that) we talked about last week.
那就是我们上周议论的淘气男孩。
六、用so，not或其它手段来省略上文或问句中的一部分或整个句义。
1.—Can Emily do this work?
埃米莉能做这件工作吗?
—I think so.
我想她行。
—I think not (或I dont think so).
我想她不行。
2.—Did you know anything about it?
这件事你以前知道吗?
—Not until you told me.
你告诉了我，才知道。
3. He has gone. No one knowsswheres(he has gone).
他走了，谁也不知道他去什么地方了。
4. Jenny knows what (is to be done)!
詹妮有办法!
七、以there be开头的句子，其主语的定语从句常可省略关联词，而there be结构作定语从句时，省略作主语的关系代词。