three-year-old 和 three years old 有什么区别

three-year-old : 三岁大的 , 三十岁
短语
Three-year-old basic orientation 三十岁基本定向
a three-year-old girl 一位三岁的女孩
Three-year-old child 三岁的孩子

three years old : 三岁,三岁大
短语
Three years old pictures 三周岁留念
Only Three Years Old 只有三岁
you are three years old 你三岁

three-year-old为形容词,比如:
my three-year-old sister
three years old为单词组合,比如
my sister is three years old.

He is a three-year-old boy.=The boy is three years old. 两者意思相同，简单地说，three-year-old 可以看做是形容词，后面必须要加上名次。 注意：three-year-old 中加了“-”，years后面的“-s ”一定要去掉。 同样的例子还有two-day off(两天休息日）

意思一样，用法不同，前者是词语，后者是语句