用于省略if的虚拟条件状语从句，有的语法学家称之为无连词非真实条件从句，主要用于正式语体。
1) 句型1：Were／Had／Should+主语=If+主语+were／had／should
例句：
Were she here, she would support the motion.=
If she were here, she would support the motion.
如果她在这儿，她会支持这个提议的。
Were he my friend，I would expect his help．=
If he were my friend, I would expect his help.
如果他是我的朋友，我会期待他的帮助。
Did I ask for help from him, he would give it.=
If I asked for help from him, he would give it.
如果我请求他帮助，他会帮助的。
Should it rain tomorrow, I should stay indoors．=
If it should rain tomorrow, I should stay indoors.
如果明天下雨，我就呆在家里。
Had you not told me about the matter, I should never have known the whole thing．=
If you had not told me about the matter, I should never have known the whole thing.
2)句型2：Were it not for+n.=If it were not for+n.
Had it not been for=If it had not been for
例句：
Had it not been for+n.=If it hadn’t been for+n.
Were it not for their help，we would be in serious trouble．=
If it were not for their help, we would be in serious trouble．
如果没有他们的帮助，我们将深陷困境。
Had it not been for their support，we couldn’t have won the election．=
If it had not been for their support, we couldn’t have won the election.
如果没有他们的支持，我们就不能赢得选举。
Arrows would continue to fly forward forever in a straight line were it not for gravity, which brings them down to earth. =
Arrows would continue to fly forward forever in a straight line if
it were not for gravity, which brings them down to earth.
要不是重力使箭落向地面，它们将永远以直线往前飞行。
Were it not for the fact that I saw it with my own eyes, I wouldn't believe it. =
If it were not for the fact that I saw it with my own eyes, I wouldn’t believe it.
若非我那时亲眼看到，我是不会相信的。
Were it not for this misfortune, he might never have been forced into the business of grinding out novels to support his family. =
If it were not for this misfortune, he might never have been forced into the business of grinding out novels to support his family.
要不是因为这一不幸变故，他可能永远都不会迫于无奈地开始流水线式创作小说养家糊口。
If it had not been for the doctor’s care, I should not be speaking to you now.
Had it not for the doctor’s care, I should not be speaking to you now.
要是当时没有医生的治疗，我现在就不能和你说话了。
If it had not been for the unusually bad weather, the rescue party would have been able to save the stranded climber.
Had it not been for the unusually bad weather, the rescue party would have been able to save the stranded climber.
要不是有反常的坏天气，抢救小组是能救出那个陷入困境的登山者的。
If it had not been for your timely help, I would have failed.
Had it not been for your timely help, I would have failed.
如果当时没有你及时的帮助，我会失败的。
She might have forgotten him completely if it had not been for recurrent nightmares.
She might have forgotten him completely had it not been for recurrent nightmares.
要不是因为反复做噩梦，她可能已经把他彻底忘了。
If it had not been for his family, He'd have gone away long ago.
Had it not been for his family, He'd have gone away long ago.
要是没有家眷拖累，他早就远走高飞了。
Nicholas might not have survived this snowstorm if it had not been for TV.
Nicholas might not have survived this snowstorm had it not been for TV.
如果不是看电视,尼古拉斯可能不会在这场暴风雪中幸存下来。
高考真题- 2015 北京
3)拓展一下，其实，让步状语从句（其中的虚拟语气用法）也可以采用无连词的句型：
Though the danger were even greater, I should feel compelled to go.
Were the danger even greater, I should feel compelled to go.
即使危险再大，我也觉得我必须去。