含蓄条件句
有时假设的情况并不以条件从句表示出来,二是暗含在上下文中,比如通过介词短语来表示。这些介词常是：but for, without, otherwis等。如：
But for your advice, I could not have done it so successfully.
如果不是听了你的建议，我就不会做的如此成功。
The change could not have taken place without the open-door policy.
没有开放的政策，就不会有如此大的变化。
I was so busy then, Otherwise, I would have told him the answer.
我当时很忙，否则，我就会把答案告诉他的。