2) －Tom must be free today.

－If so, he can help us .(he is free.)

3) －It is going to rain, isn't it?

－I hope not. (it is not going to rain.)

这种用法常见的有：How so? Why so? Is that so? I hope so. He said so.　及I suppose　/believe　/hope not. Why not?等等。

2.宾语从句中的省略

宾语从句中常省略连词that，但当有两个并列的宾语从句时，第二个that则不能省略。

1) We all know (that) the earth runs around the sun.

2) Mary said (that) she is from American and that she is 16 years old.

3. 定语从句中的省略

在定语从句中，that, which, who在从句中作宾语时可省略。

1) I haven't read the book (that/ which) you're reading.

2) The man (that/ who) your father just talked to is our English teacher, Mr. Smith.

4. 状语从句中的省略

在以when, while, once引导的时间状语从句及以if, unless引导的条件状语从句中和though, although引导的让步状语从句中，如果从句中的主语与主句中的主语

指同一对象，且从句中含有be动词时，从句中可省去"主语+be"