I. 从属连词 only, but, but that, except (that)

含有虚拟语气的分句描述的是与事实相反的情景,而含有连词 but 或 only 等的分句描述的则是事实,这一事实阻止了虚拟语气句所描述的情况成为现实:

1. Franklin would gladly have spent the rest of his days in quiet study and research. But he was a very important person now, and the country needed him for public service. 富兰克林本来是会在安静的学习和研究中愉快地度过他的后半生的，但是他当时是一个非常重要的人物，国家需要他担任公职。

2. We would have avoided making such a mistake, but we didn't follow his sound advice. 我们本来可以不犯这样的错误，只是我们没有接受他的忠告。

3. He said he would have done, only you treated him so offhand. 他说他本来会做的，只是你待他太不客气了。

4. I would have asked you, only my mother told me not to. (CGEL) 我本来要问你的，只是我妈妈叫我不要问你（要不是我妈妈叫我不要问你的话，我本来会去问你的）。

5. He would have left but that he was afraid. 要是他那时不害怕的话，他本来会离开的。