five-year-old 是复合形容词 只能作前置定语,用于修饰名词
eg.She is a five-year-old girl.
five years old 用来作表语.
eg.She is five years old.
六小时的睡眠可以有两种表达法;
six hours' sleep 这个用法相当于名词所有格
six-hour sleep ,six-hour 作sleep 的定语 ,所修饰的名词前不加 -.