[英语语法手册]虚拟语气 B
不用if的条件从句结构
在笔语中，条件从句有时可以不用连词小而把were，had或should移至主语之前。但如从句没有were，had或should，则一般不能这样做。如：
Should he come，tell him to ring me up.他要是来了，让他给我打个电话。
Were I in your position，I would go.如果我处于你的地位，我就去。
Had they not helped us，our experiment would have failed. 如果没有他们的帮助，我们的试验是会失败的。
[注]有时虚拟条件不用条件从句而用介词短语、动词不定式等来表示。如：
without contradiction nothing would exist. 没有矛盾就没有世界。
We could not have done the work well without your help.要是没有你的帮助，我们不可能做好这工作。
It wonld be a good idea to go swimming at the summer palace.到颐和园去游泳，倒是个好主意。
All this would have been impossible ten years ago.这一切在十年前是不可能有的。