问题补充：
each of them knows what the others are doing.这里的knows为什么加sshe is intersting to talk to 这里的talk 后面把to去掉为什么不对，to hear him talk in that manner，you would think he is a foreigner。talk in 这里的in去掉行吗 to hear
答案：
因为each表单数.
如果是all的话,则为all of them know...
======以下答案可供参考======
供参考答案1:
因为主语是each of them，他们中的每一个人，是单数
供参考答案2:
因为主语是each of them啊
each代表每一个，根据主谓一致原则，know要加s变成第三人称单数~~
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