 2) Why (do you) not say hello to him ?

省略表语。

1) -Are you thirsty? - Yes, I am .(thirsty).

 2) His brother isn't lazy, nor is his sister .(lazy).

4、同时省略几个成分

1) Let's meet at the same place as--- (we met) yesterday.同时省略主、谓语

 2) -Have you finished your work ? -Not yet.

-----I have not finished my homework yet.

二 并列句中的省略

1、两个并列句中，后一个分句常省略与前一分句中相同的部分。

My brother is a doctor and my sister-in-law (is) a lawyer.

省略相同的谓语动词）

When summer comes, the day is getting longer and longer, and the night (is getting) shorter and shorter. 省略相同的谓语动词

三 复合句中的省略

主句中有一些成分被省略，如：(I am) Sorry to hear you are ill.

( It's a ) Pity that he missed such a good chance.

2 主句中有一些成分被省略、而用so或not来代替

1) －Are you leaving for Beijing this Sunday?

－I think so .(I'm leaving for Beijing this Sunday.)