year old,year-old,years old,years-old的区别

year old只能是 He is only one year old。即years old的单数形式。years old就是一岁以上的了,例如She is three years old。year old 和 years old通常都作表语。首先要声明绝对没有years-old的说法,因为加上连接符以后year必须用单数。数字-year-old表示几岁,与前两个最大的区别就是它在句中只能充当定语。例如：a three-year-old boy一个三岁大的小男孩。但是不能说The boy is three-year-old。也就是说它不能充当表语,而要说 The boy is three years old。