but与however均可表示转折或对比，但有区别：

1. but 是并列连词，用于连接词、短语或句子。如：

They are poor but proud. 他们人穷志不穷。

It was a sunny but not very warm day. 那一天天气晴朗，却不太暖和。

I like cats but unfortunately I’m allergic to them. 我很喜欢猫, 但可惜我对猫过敏。

2. however表示转折时，有的词书认为它是连词，有的词书认为它是副词。之所以将其视为副词，也许是因为它像许多副词一样不仅能位于句首，而且能位于句中(注意前后使用逗号)，甚至句末(注意其前也用逗号)。

Later, however, he changed his mind. 可是他后来改变了主意。

He hasn’t arrived. He may, however, come later. 他还没有到，不过他等会儿可能会来。

He said that it was so; he was mistaken, however. 他说情况如此，可是他错了。

He said he was certain. However, he was wrong. 他说他肯定。然而他错了。

注意，说however前不能用逗号是错的，正确的表达是：当它用于连接两个句子时，它的前面不能只用逗号。但若是用于句中，它的前面和后面通常都应有逗号。如：

我们都已尽了最大的努力，不过我们还是输了。

误：We all tried our best, however we lost the game.

正：We all tried our best, but we lost the game.

正：We all tried our best; however, we lost the game.

正：We all tried our best. However, we lost the game.