（一）、省略 if 的条件从句
当从句中有 were, had 或 should 时，可省略if ，而把它们放在句首。
If I were to meet him tomorrow, I should ask him about it.-----
Were I to meet him tomorrow, I should ask him about it.
If he should fail in the experiment this time, he would try again.-----
Should he fail in the experiment this time, he would try again.
