▲没错，but后面通常是不用逗号的，但下面一句中but后面的逗号用得是恰当的：

But, three days later, a letter arrived, calling me to Groydon foran interview with the headmaster.

比较下面一句（没有用逗号）：

But a letter arrived three days later, calling me to Groydon for aninterview with the headmaster.

▲but后面使用逗号有多种原因，比如为了隔开某些前移的句子成分，或是其后跟了插入语，或是说话者希望在but后有所停顿，等等。例如：

The rain was heavy, but, contrary to our fears, our fields were notflooded. 雨很大，但是和我们担心的情况相反，我们的田地没有受淹。

But, even in Hollywood,there is no such thing as a free lunch. 但即使是在好莱坞也没有免费的午餐。

We have a lot to discuss, but,first things first, let’s have a cup of coffee! 我们有很多事情需要讨论，不过先让我们喝杯咖啡吧！

She not only deceived him but, to add insult to injury, allowed himto pay for her meal. 她不仅骗了他，更恶劣的是还让他付了饭钱。

He turned towards the open front door but, as he did so, she pushedpast him. 他转身走向开着的前门，但与此同时，她从他旁边挤过去了。

They tried to handcuff him but, despite his injuries, he fought hisway free. 他们试图把他铐起来，但尽管有伤在身，他还是奋力挣脱了。

Will you ask her? But, you know, only if she really doesn’t mind ifyou see what I mean. 你会问她吗？但是，你知道，只有她真的不介意你才能问，不知你是否懂我的意思。

Well, all right, hospitals lose money. But, on the other hand, ifpeople are healthy, don’t think of it as losing money; think of it as savinglives. 嗯，好吧，医院是赔钱。但从另一个角度看，如果人们都健健康康的，就不能认为是在赔钱，而应该想成是在救死扶伤。