eight-year-old中“-”的用法

作为形容词作定语，用eight-year-old，如:She's an eight-year-old girl.
去掉连字符，作表语，如：She's eight years old.

eight-years-old为错 应为eight years old
eight-year-old是复合形容词 当做1个形容词处理 连字符是构成复合形容词的标志

“-”是连字符，可以通过它把一长串意思相关联的单词连接在一起组成一个合成单词。eight-year-old是对的，但任何时候也不会用eight-years-old。

连一块 恩‘‘‘都做定语 修饰名词吧‘‘‘
恩 举个例子
My 8-year-old son 饿 就是做定语‘‘‘

没有eight-years-old这种形式,而eight-year-old是形容词,做定语的