Three years ago与Before three years的区别是什么?

Three years ago与Before three years的区别是什么? 请教three years later与three years ago的区别? three-year与three-years的区别是什么? three years 与three years old的区别 three years ago作文 years ago与before two years的区别. three years old 与 three-year-old 的区别 three hundred years 与 three-hundred-year 的区别? 例：three years ago/three years before 如何判别该使用什么时态? three years ago ago修饰什么 three-year-old 与 three years old区别 every third year与every three years的区别