i served the man was wearing a hat不可以,因为有两个动词可以改为i served the man who was wearing a hat. 定语从句i served the man wearing a hat.省略形式i served the man in a hat. 更省略...这两段话是因为有两个动词所以必须使用定语从句吗，为什么这个疑问句没有用助动词提问啊，who served you sir ？谢谢了这也没什么为什么,就是习惯用法罢了.记住就行了.