在含有 had/were/should 的虚拟条件句中，可以省略if, 将 had/were/should 放在主语之前，构成部分倒装。

Had it not been for the captain，the ship would have sunk. 如果不是船长，船就沉没了。

Were she my friend，I would ask her for help. 如果她是我的朋友，我就会请她帮忙。

Should he come tomorrow，I would tell you. 如果他明天来，我就告诉你。

更多例句：

Should the journeyer return, this stone would utter speech.“行人归来石应语。”

Were it to rain tomorrow, we would put off the basketball match. 如果明天下雨的话，我们将推迟举行篮球比赛。

Had she studied harder, she would have passed the exam.　假如以前她努力学习的话，她就会考试及格了。

【注意】只有当 if 从句中含有should, were, had done 的情况下（不包括 would 和could ），才可以提前并省略if。

(should, could , would ,may，might) have done —— 这些虚拟语气形式，只是用于不同人称的区别（如 should 仅限于第一人称）或意思不同，没有程度上由强到弱的差别。此外，may 可用于推测，不用于虚拟语气。