had改为has,这里用完成时,不是过去完成时.
could不代表过去,只表示虚心提问对方为什么不能用过去完成时，表他以前走了多长时间那岂不是所有“过去的动作”都要用过去完成时了？I had answered this question.I had walked for hours.I had read this book.I had finished it.I had used so many "had"s。。。。过去完成时表示过去的过去，这里用完成时就行了。