the problem, he wouldn't have mitted those mistakes.
A. If he understood
B. Had he understood
C. When he had understood
D. If he would understood
网友回答
A. If he understood
(答案→)B. Had he understood
C. When he had understood
D. If he would understood
解析：表示与过去事实相反的虚拟语气。had he understood the problem是省略的倒装形式，正常结构是If he had understood the problem。句意：如果他理解了这个问题，他就不会犯那些错误。