一、由‘数词+单数量词+形容词’可以构成一个临时的合成形容词,用来修饰名词；一般说来,这种合成形容词中间的连字符是必要的,但是也有不用连字符的,不过是属于不规范的英语,如：
He paid a visit to me with an eight-year-old girl.
He is a 20-year-old boy.
It is a five-mile-far distance from here.
二、‘数词+复数量词+形容词”通常用来做表语,表示主语的实际年龄、长度、重量、距离等,如：
The boy is eight years old.
The village is five miles far from here.