why

why除了用作疑问词之外，还可以用作导词(introductory word)，放在句首或在句中作插入语，表示惊奇等各种各样的情绪。根据上下文它可以有不同的译法（哟，啊呀，哎呀，嗨，噢，呃，啊唷等等）：

1. Why, what a bruise you have got! (COD) 啊呀,你的伤青肿得多么厉害呀!

2. Why! I thought you were in London. (JED) 哎呀！我还以为你在伦敦呢。

3. Why, what did she say? 呃,她说什么?

4. If America splits the infinitive, why, the infinitive is split, and no rule will mend it. (A. L. Guerard) 如果美国把不定式的 to 和后面的原形动词分开，呃，那么不定式就分开吧，用不着用什么语法规则来纠正它。

你提出的句子似可译为：

啊唷，你也许会对一个完全陌生的人说“Hullo!”，如果你是个英国人，要是在众人面前，这可是一件一定要三思而行的事啊。

这种用法只用于书和文章的标题：

1. Why I am Learning English?

2. How Television Programs are Born?

why引导的从句主要有：

A. 主语从句

Why he took Swiss nationality in1901 is a question that interests us. 他为什么在1901年取得瑞士国籍,这是一个使我们很感兴趣的问题。

B. 宾语从句 He asked him why he was doing that. 他问他为什么要那样做。

That is why he loved her more than us. (Dan Brown, The Da Vinci Code) 那也是他爱她胜过爱我们的原因。(《达·芬奇密码》)

D. 状语从句

No matter why she became angry with you, you should apologize to her. (SEFC, 3A, p. 95) 不管她为什么对你生气，你也应当向她道歉。

E. 定语从句, 见 appositive clause or attributive clause

I think I know part of the reason why your grandfather conspired to put us together. (Dan Brown, The Da Vinci Code) 我想我知道你祖父为什么要千方百计将我们拉到一块儿的部分原因了。(《达·芬奇密码》)

句(1)中的why是连接副词,引导宾语从句,而句(2)中的why是个缩合型的关系副词,即省略了中心词的关系副词,在意义和语法上相当于the reason why。

That's why I've come to see you. = That's the reason why[that] I've come to see you. 下面是此种用法的若干实例：

1. That's why you need the lab. 这就是你们需要实验室的原因。

2. That is why lots of people in China have healthy white teeth. 这就是为什么中国许多人都有着健康洁白的牙齿的原因。

3. That's why we were getting anxious about you. 所以,我们一直为你们担心。

4. That is why the Chinese people are making great efforts to protect their farmland and increase agricultural production. 这就是为什么中国人要下大气力来保护耕地，增加农业生产的原因了。