why 后面可以接不带 to 不定式的本质是句中隐含的成分被省略了
例如：
Why study when I already know the material by heart?
既然我已经背熟了材料，为什么还要学习呢
解析：完整的句子可能是 Why should I study when I already know the material by heart?
Why watch TV when we could play outside?
既然我们可以在外面玩，为什么还要看电视
解析：完整的句子可能是 Why would we watch TV when we could play outside?
语音朗读：