请看典型考题： __________ your address，I would have written to you. A. Did I know B. Were I to knowhttp://yingyu.chazidian.com C. Had I known D. If I should know 此题应选 C。这是虚拟语气的倒装形式。当虚拟条件句中含有 were，had，should，could等动词时，可以将if省略，然后将 were，had，should，could 等词移至句首： 1. If he should fail，he would kill himself. →Should he fail，he would kill himself. 万一失败，他就会自杀。 2. If I were you，I would do it at once. →Were I you，I would do it at once. 假若我是你，我就会马上做。 3. If I could do it，I would. →Could I do it，I would. 要是我能做此事，我一定会做。 4. If he had seen you yesterday，he would haveasked you about it. →Had he seen you yesterday，he would haveasked you about it. 他昨天要是看到了你，他就会问你这事了。 顺便说一句，以上移至句首的 had不一定是助动词，如果是实义动词也可倒装： If he had money，he would buy a car. →Had he money，he would buy a car. 他要是有钱，他就会买一辆小车。