一个英语基本英语语法问题.

题目

一个英语基本英语语法问题.
I had learned English 3 years.的倒装是
3 years had I learned English.还是3 years had I learn English.
这里假设是个过去完成时,我主要想知道在倒装后过去分词learned是保持不变,还是转为动词原形.

答案

3 years had I learned English