why引導(dǎo)定語從句，先行詞只有一個，就是The reason。如：The reason why he was late was that he got up late this morning.而why引導(dǎo)同位語從句，一般只在have no idea之類的句式中出現(xiàn)。

Why的用法

1、Why最基本的用法

Why? 為什么？--Because i want to do it! 因為我想做。

Why wait? let's leave now. 為什么要等？我們走吧。

Why should i help him--he never helps me?

他從沒幫過我，我為什么要幫助他？

Why did you leave? 你為什么離開了？

2、why引導(dǎo)的從句

I don't know why she isn't here.

我不知道她為什么不在這。

It's easy to see why he is popular.

不難看出他為何會這樣受歡迎。

My flight was cancelled, that's why I'm late.

我的航班被取消了，這就是為什么我會遲到的原因。

There is no reason why we shouldn't succeed.

我們沒有理由不成功?。ü潭ù钆洌?/p>

3、固定疑問句，表示建議

Why don't you come with us?

你為什么不跟我們一起走呢？（表示建議，為什么不……）

Why don't you finish your homework first?

你為何不先完成你的作業(yè)呢？

Why not use my car? you will fit more in.

為何不用我的車呢？你能裝更多

"Let's go out for an Italian tonight" "sure, why not"

‘我們今晚出去吃意大利菜怎么樣？’ “好啊，為什么不呢”