当虚拟条件句中含有 were，had，should，could等动词时，可以将if省略，然后将 were，had，should，could 等词移至句首：
1）If he should fail，he would kill himself.
→Should he fail，he would kill himself.
万一失败，他就会自杀。
2）If I were you，I would do it at once.
→Were I you，I would do it at once.
假若我是你，我就会马上做。
3）If I could do it，I would.
→Could I do it，I would.
要是我能做此事，我一定会做。
