had引導的虛擬語氣與倒裝

當虛擬條件句中含有 were，had，should，could等動詞時，可以將if省略，然後將 were，had，should，could 等詞移至句首：

1）If he should fail，he would kill himself.

→Should he fail，he would kill himself.

萬一失敗，他就會自殺。

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，我就會馬上做。

3）If I could do it，I would.

→Could I do it，I would.

要是我能做此事，我一定會做。