请看典型考题：

__________ your address，I would have written to you.

A. Did I know                                        B. Were I to know

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。