1. 肯定形式

Why talk so much about it? 这样大谈它干嘛?

You're fully insured, so why worry? 你已经全部保险了，那何必还担心呢?

Why ask me? How should I know? 为什么问我? 我怎么知道?

一I think I ought to tidy this place up. 我想我应该把这地方清扫一下。

一Why bother? 何必麻烦呢?
 

2. 否定形式

Why not try again? 干嘛不再试呢?

Why not go now? 现在就去好不好?

Why notjust wait and see what happens? 何不静观其结果呢?

Why not try to expand your story into a novel? 你怎么不把你的故事扩展成小说呢?

Why not wait till the winter sales to buy a new coat? 为什么不等到冬季大贱卖时再买一件新外衣呢?
 

3. why not单独使用

I'm not going to work today. 一Why not? 我今天不去上班了。——为什么不呢?

Let's eat out tonight. 一Yes, why not? 我们今晚出去吃饭吧。——对，为什么不呢?

Are you really going to sue them?一Yes, why not? 你真的打算控告他们吗?——是的，为什么不呢?
 

4. 与非省略句的替换

对于一般现在时而言，它可与非省略句转换：

你为什么不找点事干，比如给我擦洗汽车？

正：Why not find something to do, like cleaning the car for me?

正：Why don't you find something to do, like cleaning the car for me?

对于一般过去时而言，它不能与非省略句转换——只能用非省略形式，不能用省略句：

为什么你没告诉我约翰已经离开了你?

正：Why didn't you tell me John had left you?

误：Why not tell me John had left you?