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used in questions to ask the reason for or purpose of something （用于问句）为什么，为何 -
used in questions to suggest that it is not necessary to do something （反问，表示不必）何必 Why get upset just because you got one bad grade? 何必因为一次成绩不好就想不开呢？ Why bother to write? We'll see him tomorrow. 还费事写信干什么？我们明天就见到他了。
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used to give or talk about a reason （说明理由）为什么，…的原因 That's why I left so early. 这就是我早早离去的原因。 I know you did it—I just want to know why. 我知道这是你干的，我只是想知道为什么。 The reason why the injection needs repeating every year is that the virus changes. 每年需要重新注射的原因是这病毒经常变化。
Express Yourself Giving reasons, justifying a choiceGiving reasons, justifying a choice 对选择作出解释 In various exams, you are asked to make a choice and give reasons for it. In conversation or in a meeting, you need to explain and justify your decisions: 在各种考试中需要作出选择并给出理由，在谈话或会议上需要对决定加以解释，可用以下表达方式： There are two main reasons why I think it’s the best option: first, there's the cost and second, the quality. 我认为这是最好的选择，主要原因有两个：第一是成本，第二是质量。
I think/believe it's the right thing to do because it gives everyone a fair chance. 我认为这么做是正确的，因为这给予每个人公平的机会。
I would choose the newer one on the grounds that it will last longer. 我会选择较新的那个，因为它更加耐用。
Of the three houses, the largest one seems to me to be the best, because they need the room. 这三栋房子中，最大的那栋在我看来是最好的，因为他们需要这么大的空间。
My choice would be number 3, simply because it's the clearest design. 我的选择是 3 号，原因就在于它的设计最清晰。
Word OriginOld English hwī, hwȳ ‘by what cause’, instrumental case of hwæt ‘what’, of Germanic origin.
Idioms
why ever
why not?
used to make or agree to a suggestion （提出或赞同建议）为什么不呢，好哇 Why not write to her? 为什么不给她写信呢？ ‘Let's eat out.’ ‘Why not?’ “咱们到外边吃去吧。” “好哇。” Why don't we go together? 我们为什么不一起去呢？