but 作为连词在大多数情况下都用作并列连词，但有时在一些固定结构中也可用作从属连词。
一、用作并列连词
作为并列连词，but 可以连接两个并列的分句或者两个并列的成份，译为“但是”，“可是”。能表示下列意义：
1、表示意义转折。例如：
they are tired but happy.
i was going to write, but i lost your address.
not one but two!
2、表示对比或对照，通常与否定词连用。例如：
he is not good at maths, but he is good at english.
he scarcely knows french, but he speaks english perfectly.
3、出现在句首或句与句之间，表示异议，惊奇或吃惊。如：
a:i have won the long jump.
b:but that's wonderful!
4、用在表示歉意的词语之后，引起一个句子，长不能译为“但是”，可以不译出。如：
i'm sorry, but i can't come.
excuse me, but can you tell me the way to the library?
固定短语：1、not only … but (also) … 这是一组关联并列连词，表示其所连接的成份在意义上后一项对前一项的补充和引申，但侧重点在后一项上，意思是“不但……而且……”。但使用这一组关联并列连词必须注意两点：
(1)、当 not only … but also 连接两个并列成份作主语时，其谓语动词的形式按照就近原则来确定。
(2)、当 not only … but also 连接两个并列成份时采用部分倒装形式，即倒装 not only 分句，而 but also 分句则采用陈述句语序。如：
△ not only the mother but also the children are sick.
△ not only is he interested in the subject but also all his students are beginning to show interested in it.
有时，but 和 also 可以被分隔使用。如：
she was not only compelled to stay at home but she also forbidden to see her friends.
有时，but also 能被 but … as well 所取代。如：
he can not only speak english but french as well.
2、but then, 意思是“不过，另一方”，相当于 nevertheless, on the other hand”。如：she speaks english fluently, but then she did live in london for four years.3、not that … but that … 引导两个并的原因状语从句，that 相当于 because, 意思是“不是因为 …… 而是因为 ……”。如：
i haven't worked out the maths problem, not that i have no time for it, but that i can't work it out. 有时 not that … but that … 结构可单独出现。如：
not that i won't go, but that i don't know which is the way to go.
二、用作从属连词。
but 在一些固定结构中作从属连词，引导状语从句和名词从句。
1、引导条件状语从句。
but that 可以用来引导条件状语从句，意思是“倘若，要不是”。《牛津现代高级英汉双解词典》和一些语法书上将此结构中的 but 看成介词，that 则引导了特殊的介词宾语从句，但是《牛津现代高级英汉双解词典》和章振帮先生所著《新编英语语法》等认为 but that 为复杂从属连词， but that 从句为条件状语从句，本人倾向于后一种看法。 that 从句一般不作介词补足成分，将 but that 理解为复杂从属连词，有助于确切理解从句 but that 的真正意义， but that 从句用陈述语气，而主句则常用虚拟语气，but that 从句可以同否定的 if 从句互换。
例如：
but that you lent him a large sum of money, he would have gone bankrupt.
此句可改为：
if you hadn't lent him a large sum of money, he would have gone bankrupt.
有时 but that 从句出现于含有 not 的主句之后，意思是：“只有…… 才 ……”。例如：he will not find the answer but that he studies it thoroughly.
he will not come but that he is asked.
2、引导比较状语从句。
用 no sooner … but … 结构引导比较状语从句，but 相当于 than 如：
it no sooner started raining but it stopped.
3、用 not but that (what) 结构引导让步状语从句，意思是“虽然，诚然”。如：
he is very strong — not but that he will catch cold sometimes. 意思为“他身体很强壮，虽然有时会患感冒”。
i've never walked that far, not but what i could do it if i tried. 意思为：我从来没走那么远，虽然我要走还是可以走。
4、引导结果状语从句。
(1)not so … but (that/what)… 或 not such a … but，意思为“不是如此……以致于不”。其中 but 的意思是接近于“that … not …”。例如：
he is not such a fool but that he can see through mary.
(2)否定句＋but(that)＋从句，意思为“每当……总是……”，其中 but 相当于“that…not …”，通过双重否定来表达肯定的因果关系。例如：
i never go past my old school but i think of the happy years i spent there.
意思为：“每当我走过我的母校时，我都想起我在那儿度过的快乐岁月。”
5、引导名词性从句，常用于下列结构中：
i didn't doubt/denty/question but that…
there is no doubt/question but(that)…
这些结构中的 but 或 but that 并无实在意义，相当于连词 that 。例如：
there is no doubt but he is the quilty one，意思为：“毫无疑问，犯罪的就是他”。
在实际应用中，but 用作连词和介词容易混淆。如果 but 被看作连词，其后的人称代词可以被看成是一个省略的谓语部分的主语。应该是主格形式。例如：
everyone lost fainth in the plan but she.
no one found the answer but she.
更为常见的是 but 用作介词，这样其后的人称代词“she”可改为格“her”。
但是，一般说来，but 和其后的人称代词如果置于谓语动词之前，人称代词用主格形式。如：
evryone but she lost fainth in the plan.
no one but she found the answer.
至此，本文较为详细地归纳了 but 作为连词时的用法，深入地探讨了 but 作为并列连词和从属连词及其固定短语的意义和用法，并配以实例说明。这样对于我们理解含有 but 的句子尤其是含有 but 的双重否定句将有一定作用。
参考文献：
(1)robert b lostello\ 1995.《random house webster's college dictionary》.\ new york:random house.
(2)章振帮1990《新编英语语法》上海：上海译文出版社
(3)李此达1994《牛津现代高级英汉双解词典》牛津大学出版社课堂练习的几个设计
