助词,情态动词的后面 (陈述语序）,省略倒装的情况下放在S（主语）的后面
Were I Tom,I would refuse.如果我是汤姆,我会拒绝.
I will go,should it be necessary.假若有必要,我会去的.
Had it not been for the bad weather we would have arrived in time.若不是天气坏,我们就准时到达了.