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but for“要不是”，本身就含有对于已发生的事情的一种假设，所以表示的是过去的某一个状态。例句：But for his help,i couldn't have made such big progress.要不是他的帮助，我不会取得如此大的进步！
but for相关例句
I'm changing my email address but for the meantime you can use the old one.
我要更换电邮地址，不过那个旧的暂时也还可以用。
I'm not afraid for me, but for the baby.
我担心的不是自己，而是婴儿。
I was sure she knew the answer but for some reason she hung back.
我敢保证她知道答案，但不知为什么她不敢说出来。
There's still another ten minutes of the game to go, but for practical purposes it's already over.
比赛虽然还有十分钟，但实际上等于已经结束了。
But for you, they might have given us the slip.
要不是你，我们可能已经被他们甩掉了。
虚拟语气
虚拟语气表示说话人的主观愿望、猜疑、建议或与事实不符的假设等，而不表示客观存在的事实。虚拟语气是由句中的谓语动词的特殊形式表示出来的。如：
If I were a bird，I would be able to fly in the air.
如果我是一只小鸟，我就能在空中飞行。
I wish I could pass the examination.
我希望我能通过考试。
Hope you succeed!
祝你成功！
If only we had been college students last year!
上一年我们要是大学生该多好！