(1) 这些结构都可以用来表示某一具体情况使一切发生了变化，意思是“若不是……”，“若无……”。(另见but for条)

• If it were not for his wife's money, he'd never be a director. (=Without his wife's money...)
  若没有他妻子的钱，他绝不会成为一名导演。

 (2) 如要说过去的情况，则要用“If it had not been for...”结构。

• If it hadn't been for your help, I really don't know what I'd have done.
  若没有你的帮助，我真不知我会做出些什么事。

 (3) only for通常用于口语，意思同but for。

• Only for your timely help, we wouldn't have succeeded in finishing this task.
  如果不是你的及时帮助，我们就不会完成这项任务。

 (4) only that意思同but for the fact that, with the exception that, were it not that，意思是“要不是(因为) ……；只是……”，引出的从句后置，前面的主句中一般用虚拟语气。

• I would be very glad to go with you only that I am too busy. (=If I were not too busy, I would...)
  要不是我太忙了，我会很高兴与你同去。
• He would probably do well in the examination, only that he gets rather nervous.
  若不是有点紧张，他可能会考得很好。