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、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。

　　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、在direction(方向)，way(方式)，distance(距离)，time(时间)，times(倍数)等后面所接的定语从句中常省略that，which，in which。

　　1. The direction (in which) we move a body can be changed.

　　我们移动物体的方向是可以改变的。

　　2. The distance (which或that) light travels in one second is 300，000 kilometers.

　　光每秒走的距离是30万公里。