请教three years later与three years ago的区别？

请教three years later与three years ago的区别？

1) three years later 以过去某一点为参照，意思是：“三年后”如：In 1986, I went to Beijing, and three years later, I returned to my hometown to be a teacher in a primary school.
2) three years ago 以现在为时间点为参照，往回数三年，就是“三年前”的意思。句子用一般过去时。如：It is 2011,I am a college student now in Nanjing university,but three years ago, I was a middle school student.
另外注意：以现在为起点，往后数的三年，我们应该用 In three years，即“三年后”或“今后三年内”。如：I will buy a new house in three years.
I will finish doing this work in 3 hours.

three years later中的later是表达：以现在为起点，三年以后； three years ago中的ago是表达：以现在为起点的三年以前。