我想问英文中数字什么时候与连接符连用 比如说到底用 three-year-boy 还是 three years boy

我想问英文中数字什么时候与连接符连用 比如说到底用 three-year-boy 还是 three years boy a three－year－old boy,可以说成a three years old 是two three-year boys还是two three-year boy,谓语是什么数? three-year-old boy 这样的前面用a还是an?两种我都看过.. three years old 与 three-year-old 的区别 three-year-old 与 three years old区别 three hundred years 与 three-hundred-year 的区别? three-year与three-years的区别是什么? At last the three-year-old boy was____(救） this is the boy( ) father died three years ago 定语从句,句子分析到底分析什么啊?例子：A 15-year-old boy who went missing three 1. The boy left school three years ago.(同意转换) = The boy ____