A 如果我（早）知道这计算机程序，就能节约大量短时间和精力了虚拟语气，表示与过去事实相反的情况。其句型为“ if + 主语 + 动词的过去完成式，主语 +would / could / should / might + have + 动词的过去分词”。这道题里从句Had I known about this computer program是省略了if 然后倒装的=If I had known about this computer program. 主句中a huge amount of time and energy和save是被动关系，所以用would have been saved....
