without, otherwise, but for, but引导的短语或句子有时相当于一个虚拟条件句.假设的情况并不以条件从句表示出来,而是暗含在上下文中,通过介词短语来表示.
使用到这些词组的句子中,虚拟语气的用法一般是：对现在的虚拟：would+do；对过去的虚拟：would(not) + have done.
比如：But for your reasonable advice,i wouldn't have fininshed my paper excellently.Without bring in him as the PM(project manager项目经理)of our company,the important program would have failed.
I should study more hard,otherwise i would have kepet with the most outstanding sutdent in my class in last exam. Your mother would go with you,but she has to take care of your elder brother