虚拟条件句的从句部分若含有were, should 或had，可省略if，再把were, should或had 移到从句的句首，使用倒装句型。例如：
Had I known your address, I would have written before.= If I had known your address, I would have written before. 我要是知道你的地址我早就给你写信了。
Were they here now, they could help us. =If they were here now, they could help us. 他们现在在的话，就会帮助我们了。
注：若条件从句为否定句，否定词not应置于主语之后，而不能与were, should, had 等缩略成Weren’t, Shouldn’t, Hadn’t而置于句首。另外，有时省略if后提前的had不是助动词。如：
Had I time, I would come. 假若我有时间，我会来的。(=If I had time…)