the doctor asked what he had been eating为什么引语要用过去完成进行时

间接引语的规矩：
原句是过去进行时,改为间接引语就要用过去完成进行时.
原句：The doctor asked,"what were you eating?"
原句是过去时,间接引语就要用过去时：
原句：He said,"I am interested in music."
间接引语：He said that he was interested in music.
等等,还有好多类似的规矩.