but 一词既可作连词、介词、副词用，又有一些习惯搭配和固定用法，这里对but做连词用法做一归纳。
1、表示转折关系，意为“但是，然而，可是”，所连接的成分意思相反或相对，使其前后的词、短语、分句相互对照。例如：
Our school is small but beautiful.我们学校虽然小但很漂亮。（连接两个形容词，意思相对）
The car is very old but it runs very fast.虽然小汽车旧了，但跑得很快。
She is young but very experienced. 她虽然年轻但经验丰富。
It was a sunny but not very warm day. 那一天天气晴朗，却不太暖和。
He drives not carefully but slowly. 他开车不是很小心，而是开得很慢。
按英语习惯，表示转折的but不能与表示让步的though或although套用，如：
他们虽是孪生，但是相貌却完全不同。
误：Although they are twins, but they look entirely different.
正：They are twins, but they look entirely different.
正：Although they are twins, they look entirely different.
2、并列连词but用于not…but结构，意为“不是……而是”。例如：
What I want is not this one, but that one. 我想要的不是这个，而是那个。
Bamboo is not a tree, but a kind of grass. 竹不是一种树，而是一种草。
Travel to the moon is not a dream but an actual happening. 登月旅行已不是梦想而是现实事情了。
3、 用于 excuse me, I'm sorry表示歉意的话语之后，表示谢绝或不赞成。例如：
I'm sorry, but I disagree with you. 对不起，我不同意你的意见。
Sorry, but we're behind schedule. 抱歉，我们落在计划后了。
I'm frightfully sorry, but I can't see you today. 太对不起了，我今天不能见你。
Excuse me, but I don't think that's quite true. 很抱歉，我认为这与事实略有出入。
4、用于两个并列的分句之间，例如：
I never go past that house but I think of my miserable life in the old society. 我走过那所房子时，没有一次不想起我在旧社会所过的悲惨生活。
This isn't a good one but it will answer. 这不太好，但可以将就用。
He looks honest, but actually he's a rogue. 他看起来很老实，实际他是一个坏蛋。
She has had no answer to him but he gave no answer. 我向他说了早上好，但他没有回应。
The ice remained, but there was no water underneath. 冰还在，但下面却没有水。
Mrs. Brown was about to begin, but Jennie spoke first. 布朗夫人正要开始讲话，珍妮先讲了。
At first he was a little shy, but now he acts more natural. 开始时他有些腼腆但现在他表现得很自然了。
There was a little trouble at first, but things were soon quiet. 起初有点小麻烦后来情况就平静了。
It never rains but it pours. 不雨则已，一雨倾盆。
5、并列连词but有时可用于表示异议、惊奇或吃惊等。如：
“I'm getting married.” “But that's wonderful!” “我快要结婚了。”“啊，那太好了!”
6、并列连词but用于否定词语(尤其是never，有时也可能是not, no, hardly等)后，表示without the result that...或without it also being the case that...，其意为“而不产生…的结果；而不同时也…”，结合其前的否定词通常译为“一…就”“每…都会”“一…总会”“没有…而不”。如：
It never rains but it pours. 不雨则已，一雨倾盆(指事情不发生则已，一旦发生则接踵而至，有时译为“祸不单行”)。
I never see her but I want to kiss her. 我每次见到她都想吻她。
You cannot look into the index but you will find the word. 你一查索引，就可以找到那个词。