1. but 从根本上是个连词，连接两个对等的部分，最常见的意思是指：但是、不过，用来表示转折或是相反的语气
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. 句子中相同的部分可以省略，用法同 and
My friend Lisa is smart but shy. 我的朋友 Lisa 很聪明，但是很害羞
This snack is good to eat but hard to find. 这一款点心很好吃，但很难找到
The restaurant is very nice, but the food there is too expensive. 这件餐厅很棒，不过那边的事物太贵了（通常只有当 but 连接两个句子时，前面需要加上逗号）
but 的其他用法
一、but + 短语：作句子的状语
He tried to save it, but in vain. 他试图挽救它，但没有成功。 but + 介词短语作结果状语
He worked very hard, but without success. 他工作很努力，但没有成功， but + 介词短语做结果状语
She glanced qucikly about, but seeing only the empty room. 她迅速扫视了一下四周，但只看到了空房间。but + ing 分词作结果状语
二、but that 的用法
but that 1:
请问下面两句中 but that 的意义和用法是否相同?
Nothing would satisfy the child but that I place her on my lap.
Nothing is so hard but that it becomes easy by practices.
答：上面两句中的 but that 都是从属连接词, 但意义和用法是不同的。在第一句中 but (that) 的含义相当于 except that, 引出的是例外分句, 全句的意思是: 除了把这孩子放在我的膝上之外, 什么事也不能使她满意。而第二句中的 but that = that…not, 全句的意思是: 没有什么事通过练习不会变为易事(什么难事通过练习都会变为易事) 。这个句型的特点, 是主句总是个否定句。请看下面的例句:
No man is too old but he may learn. 不管多老, 都可以学习。 but = that … not
I am not such a fool but that I understand you. 我绝不会蠢得连你都不了解。
No task is so difficult but we can accomplish it. 没有什么困难的任务是我们不能完成的。but = that … not
Nowadays diamond is not such a hard material but that it can be drilled through. 金刚石在今天已不是硬得不能钻透的材料了。
上述句子还不懂可以先看上面的解释，然后再回头来看
but that 2:
I’d have come with you but that I am so busy. 请问 but that 的词性及其含义：
答：but that 如同 except that, in that 一样, 应看成是复合从属连接词 (complex subordinator), 而不要看成是介系词 + that 的结构。此种用法的 but that 通常可以译为: “要不是” (= except that / if … not) 。
The old woman would have been drowned but that a passer-by dragged her out of the water. 要不是一位过路人把这位老太太从水里拉上来, 她就淹死了。
He would have helped us but that he was short of money at the time. 要不是他当时没有钱, 他就帮助我们了。
I would have failed but that you helped me. 要不是你们帮助我，我就会失败。= I would hava failed if you don’t help me.
请注意, but that 子句本身所描述的情景是真实的, 应当用陈述语气, 但是, 由于 but that 在语意上表示 “假如它所描述的真实情景不存在的话”, 因此主句常常提出与事实相反的情景, 所以主句大都用假设语气。
but that 3:
下面句子中的 but 是什么词类?
It never rains but it pours.
答：这里的 but 是 but that 的省略形式。but that = except that (若不是, 除非) 。因此 but (that) 是个从属连接词, 引出例外子句。
当主句为否定句时, 往往表达 “如果不⋯⋯ 就不会”、“ 若⋯⋯, 就一定” 的意思, 而且在固定格式里常常省略 that 。在这种情况下, but (that) 相当于 without + V-ing
It never rains but it pours. (= It never rains without pouring) 不下雨则已, 一下雨就是倾盆大雨。
You cannot look out but you will see it. (= You cannot look out without seeing it.) 你只要往外看, 就能看见它。
He never comes but that he may scold us. 他不来则已, 一来就骂我们。
He never reads a book but he digests. 他读书必融会贯通。
I never hear such a story but I laugh. 我一听到这种故事就发笑。
I never think of summer but I think of my childhood. 每当我想起夏天, 我就想起我的童年。
I never hear Gigli records but I think of my childhood singing-teacher. 每当我听吉格里的唱片时, 就会想起童年时的唱歌教师。
but that 4:
请问下面句中的 but that 是什么词, 具有什么含义?
I am not sure but that spring has arrived. 我不确定，春天没有到来。
答：but that 用在 to know, to be sure, to believe, to think, to say, to imagine 等动词之后, 其含义是: that… not 。所以现在有一种倾向用 that… not 来代替 but that 。例如:
I don’t know but that he may come here tomorrow. 我不知道，他明天可能不到这里来。
Who knows but that we may make friends with these foreigners?谁知道我们不会同这些外国人交朋友呢? (我们也许会同这些外国人交上朋友的。)
Who knows but that we may make a fortune? 谁知道我们不会发财呢?
I could hardly believe but that it was all real. 我很难相信这不是真的。
三、but 用作连词，放在否定词或疑问词之后，作从属关系，表示否定意义，可相当于 that not.
Never a month passed but she writes to her parents. 她没有一个月不给她双亲写信。
四、but / but that 用于否定词 doubt, question, deny 等之后，没有实在意义，只相当于关系连词 that.
There’s no doubt but he is a thief. 毫无疑问，他是一个贼。
I don’t doubt but that you are surprised. 我不怀疑，但你会感到惊讶。
在肯定句中, doubt 之后的子句要使用连接词 whether 或 if :
He doubled whether they would be able to help. 他拿不准他们是否能帮忙。
I doubt if he’s honest. 我怀疑他是否诚实。
在否定句中, doubt 之后的子句必须使用连接词 (but) that:
He never doubted that they would succeed. 他从不怀疑他们会成功。
I don’t doubt that they’ll accept at once. 我不怀疑他们会马上接受。
He didn’t doubt but that (= that) snow would fall. 他不怀疑要下雪了。
doubt 用作名词时, 情况亦是如此:
There is no doubt that if she had not lost her rudder she would have won the race easily. 如果它没有失去船舵的话, 它可能会轻而易举地赢得这次比赛的胜利的。
There’s no doubt but that he’s the guilty one. 毫无疑问他有罪。
I have no doubt but that he’ll come. 我想他会来的。
五、but 用作介词，表示 “除了，相当于 besides, except.”
No one knows him but she. 除了她，没有人认识她。
六、but 用作副词，相当于 only, 常译为 “只不过，仅仅”
He finished his homework but ten minutes ago. 他十分钟前刚完成作业。
七、buy 用作代词，表示从属关系，在否定句中常相当于 who 例如：
There is no one but likes to help him. 没有人喜欢帮助他。
but 的高级用法
1. 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。
I never/hardly/scarcely/do not go past his grave but I think of him. 每次我路过他的墓，就会想到他。因为 but 是否定，所以这是双重否定，表示彻底肯定。
we never go to match but it rains. 我们一比赛，天就下雨：
He never passed one day but he phoned his girlfriend. 没有一天他不给女朋友大电话。
He never speaks but she contradicts him. 他一讲话，她就顶撞他
It never rains but it pours. 不下雨则已，下则倾盆。
2. 完全肯定词加 but 表示否定的态度；
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. 除了绘画，他什么都不谈
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？
省略前面的完全肯定词
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.
3. 完全否定词加 but，表示肯定
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
如果用肯定词 anything + but，是彻底否定
with anything but his own talent, he made himself a prisoner. 他吃喝嫖赌都干，就是不利用自己的才能，结果锒铛入狱。
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。
5. but for 的用法
but for 用法 1:
(a) But for a half or so for lunch I am here all day.
(b) But for my brother’s help I would not have finished the work. 句 (a) (b) 中的 but for 意义是否相同?
在句 (a) 中 but for 表示 “例外” (除 ⋯ 之外), 在意义和用法上近似于 except for, 但没有后者用得普遍。在句 (b) 中 but for 表示“ 否定的条件”(要不是, 如果没有), 而且这种否定的条件是与事实相反的, 因此, 句子的动词要用假设语气。请注意下列各句中 but for 在意义和用法上的区别:
But for John they all died. 除约翰外, 他们都死了。
But for John they would all have died. 如果没有约翰, 他们就都死了。 类似例子:
But for the sun’s head, nothing could live… 如果没有太阳的热量, 任何生物也不会存活。
But for the rain we should have had a pleasant journey. 要不是那场雨, 我们的旅游会是很愉快的。 But for the storm we should have arrived earlier. 要不是那场暴风雨, 我们本会早点儿到的。
But for your coming, I should have been very lonely. 如果那时不是你来了的话, 我是会非常孤独的。
but for 用法 2:
but for 除了表示“倘若没有”之外, 也可以表示 “除⋯ ⋯之外” 的意思。请看下面的实例:
But for John they all died. 除了约翰大家都死了。
But for a half or so for lunch I am here all day. 除了大约半小时的午饭时间之外, 我整天都在这里。
But for you everyone has helped. 除了你之外, 大家都帮忙了。
but for 用法 3 (but for/ except for):
but for 意为 ‘要不是’，用于假设语气
She would leave her husband except for the children.
except for 表示例外, 作 “除了” 解时, 常常与陈述语气连用。例如:
Except for me, everyone was tired. 除了我之外, 大家都很疲劳。
Except for John, all died. 除了约翰, 其余的人都死了。
Except for John, the whole class passed the test. 除了约翰, 全班同学都通过了考试。
Except for us/ourselves, he whole village was asleep. 除了我们之外, 全村的人都睡着了。
当 except for 表示否定的条件, 作 “要不是” 解时, 则如同 but for
Except for you, I should be dead by now. 要不是你, 我现在已不活在人世了。
Except for John they would all have died. 要不是因为约翰, 他们大家都死了。