问题补充：
him well, I would have invited him to my birthday party.
A. Had I known
B. Should I know
C. If I know
D. If I knew
答案：
(答案→)A. Had I known
B. Should I know
C. If I know
D. If I knew
解析：考查虚拟语气。根据句意考查与过去事实相反的虚拟语气，故从句用had + v-ed。Had I known = If I had known。句意：如果我很了解他的话，我将邀请他参加我的生日晚会。
