but 既可以用作并列连词，也可以单独使用。无论哪个用法，后面均可以跟一个复合句。也就是说，but 后跟before, after, when, while, as soon as, if, because 等引导的从句，完全可以。也就是说，在它并列的后面那个分句，是个复合句而已。这样的用法很多。

The boat leaves Liverpool on the fifteenth, but before we set sail for New York I have to spend a few days in Scotland. 

船十五号离开利物浦，但是开往纽约之前，我得在苏格兰呆上几天。

But before you start,would you do this for me? 

你在开始整理之前，可以先为我做件事吗?

He's reluctant to begin, but when he does you should see him go into action. 

他不愿意开始，但他一旦开始了，你就会看到他干得十分卖力。

I used to believe in strict discipline for children. But after I had my own, I had a change of heart. 

过去我认为对孩子必须严格，但我有了自己的孩子后，我的看法改变了。

I don't think it will happen, but if it should what shall we do? 

我认为那件事不会发生，但如果真的发生，我们该怎么办？

【跟踪解答】

这个问题并不复杂，不要考虑太多了。其实，我们把上面的句子中but 后面的before, after, when,if 等连词放在句子末尾，你就明白了。

The boat leaves Liverpool on the fifteenth, but I have to spend a few days in Scotland before we set sail for New York. 

船十五号离开利物浦，但是开往纽约之前，我得在苏格兰呆上几天。

But would you do this for me before you start? 

你在开始整理之前，可以先为我做件事吗?

He's reluctant to begin, but when you should see him go into action when he does. 

他不愿意开始，但他一旦开始了，你就会看到他干得十分卖力。

I used to believe in strict discipline for children. But I had a change of heart after I had my own. 

过去我认为对孩子必须严格，但我有了自己的孩子后，我的看法改变了。

I don't think it will happen, but what shall we do if it should? 

我认为那件事不会发生，但如果真的发生，我们该怎么办？