如何把He said "I have studied English since I was a boy."改为间接引语?为什么?

问题描述:

如何把He said "I have studied English since I was a boy."改为间接引语?为什么?

He said he had studied English since he was a boy.
有关现在完成时直接改间接的方法:
1.将引号里的现在完成时改成过去完成时
2.人称、时间状语等作相应变化