Had he not been so busy taking diamonds,he would have felt the blow.
1。例句中，为何要把had提到句首，如此有哪些用途？
2。还想问一下老师， have done , 语气由强到弱，如此排列对吗？
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. 假如不是船长，船就沉没了。
Were she my friend，I would ask her for help. 假如她是我的朋友，我就会请她帮忙。
Should he come tomorrow，I would tell you. 假如他明天来，我就对你说。
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. 假如明天下雨的话，大家将推迟举行篮球比赛。
Had she studied harder, she would have passed the exam. 倘若以前她好好学习的话，她就会考试及格了。
【注意】只有当 if 从句中含有should, were, had done 的状况下（不包含 would 和could），才能提前并省略if。
have done —— 这类虚拟语气形式，只不过用于不同人称有什么区别（如 should 仅限于第一人称）或意思不同，没程度上由强到弱的差别。除此之外，may 可用于推断，不需要于虚拟语气。