/waɪ; US hwaɪ ||; hwaɪ/adv1 for what reason 为什么: Why was she so late? 她为什么这么晚才到？I wonder why they went. 我不知道他们为什么要去。‘I'm not staying any longer.’ ‘Why not?’ “我不想再待下去。”“为什么呢？”
2 used for giving or talking about a reason for sth （用于提供或谈论原因）: The reason why I'm leaving you is obvious. 我要离开你的原因是显而易见的。I'm tired and that's why I'm in such a bad mood. 我累了，这就是为什么我的情绪如此不好。
why ever used to show that you are surprised or angry （用于表示惊讶或生气）究竟为什么:
Why ever didn't you phone? 你究竟为什么没打电话来？ why not? used for making or agreeing to a suggestion （用于提出建议或表示同意）何不，有何不可:
Why not phone her tonight? 今晚何不给她打个电话呢？‘Shall we go out tonight?’‘Yes, why not?’ “我们今晚出去好吗？”“好的，干吗不呢？”