核心问题
Why, you might be saying “Hullo!” to a total stranger, a thing you would certainly think twice about before doing in public, if you were English.
请问句中 why 的意义和用法。
词典解答
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）：
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Why, what a bruise you have got! (COD) 啊呀,你的伤青肿得多么厉害呀!
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Why! I thought you were in London. (JED) 哎呀！我还以为你在伦敦呢。
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Why, what did she say? 呃,她说什么?
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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。