定语从句why的用法和例句总结如下：

1. why可引导限制性定语从句，修饰表示原因的先行词（reason），在从句中作状语，常可省略

The reason (why) she succeeds is obvious. 

她成功的理由非常明显。

【why修饰原因（the reason），在从句中作状语，可省略】

Give me one good reason (why) I should help you. 

给我一个我该帮助你的好理由。

The reason (why) he did it is unknown. 

他做这事的理由没人知道。

The reason why we are holding this meeting is to meet each other.

我们举行这次会议的原因是为了彼此见面。

There are two main reasons why I think it's the best solution.

我认为这是最好的解决方案的原因有两个。

Did you know the reason why she suddenly changed?

你知道她突然改变的原因吗？

There are several reasons why she isn't fond of Tom.

她不喜欢汤姆的原因有几个。

2. why经常可与for which替换使用

This is the reason why I quit my job. 这就是我辞职的原因。

= This is the reason for which I quit my job.

以上就是定语从句why的用法和例句总结，希望对你有所帮助。你可能也需要进一步学习：the reason why和the reason that区别