but 根本上 是个连词，连接两个对等的部分。
I would have written before, but I have been ill. 我以前本可以写，但我生病了。
They see the trees but (they do not see) the forest.
Not you (are to blame) but I am to blame for this mistake. 所以可以理解这里的主谓一致，用就近原则。
He tried to save it, but in vain. but+介词短语作结果状语
He worked very hard, but without success. but+介词短语做结果状语
She glanced qucikly about, but seeing only the empty room. but+ing分词作结果状语
如果想表达“每当.....总是”，“即便在非常不利的情况下，也能” 这样的意思。 可以这样写，如：
第一句用否定： I never/hardly/scarcely/do not go past his grave. 第二句用but： but I think of him.
I never/hardly/scarcely/do not go past his grave but I think of him.
we never go to match but it rains.
He never passed one day but he phoned his girlfriend.
He never speaks but she contradicts him.
It never rains but it pours. 不下雨则已，下则倾盆。
完全肯定词： anything everyone, everywhere, everything who whose all
He talked of anything but painting. 否定painting 他不讨论画画的事。
（he never talked of anything but painting. 请翻译这句什么意思）
He took everhthing but that book. 对那本书彻底否定的态度， 他不拿那本书。
Everyone has known it but you. 你太蠢了，只有你不知道。
All but he had fled. 别人都跑，只有他没跑。
Who but a fool would do such a thing? 除了蠢货，还有谁会做这种事？
whose faults is it but hers? 不是她的错，还能是谁的错？
Our world is but a small part of the cosmos. 我们的世界仅仅是宇宙的一小部分而已.
完全否定词： nothing none, nobody, nowhere
you will have nobody but youself to blame for this mistake. 所有人都不赖，就赖你。
No one but me knows what really happened. 只有我，别人都不知道。
She looks nothing but well. 看上去还有些健康。
With nothing but his own talent, he made himself rich and famous. 什么都不靠，只靠他的才能
with anything but his own talent, he made himself a prisoner.
4 but 的虚拟语气
but for 加短语，主句用虚拟语气
but for the rain, we would have arrived on time. 要不是下雨，
But for your coming, I should have been very lonely. 要不是你来了
but that 加 从句
but that he was prevented, he would have finished his design. 他本来可以完成设计，但被阻止了。
He would have said no, but that I was afraid. 他害怕了，原本他会拒绝了。
最后，经常使用的“not only....but also”表示递进
结构上要注意： not only 是副词， but （also）是连词
Air not only occupies space but it also has weight. 空气不仅占据空间而且有重量。
I will not only reveal it, but ( I will） let the whole world know it. 我不仅要说出去，还有让全世界都知道。
Not only you but also I am planning to go.