什么时候必须用but不能用however

什么时候必须用but不能用however

1. however作副词用时，表示“然而；但是”，可以位于句首、句中和句末；位于句首时，要用逗号与句子其它部分隔开；位于句中时，其前后都要用逗号；位于句末时其前用逗号分开。 例如:
She felt ill. She went to work, however, and tried to concentrate.
她病了。然而她照旧去上班，并且尽力集中精神工作。
His first response was to say no. Later, however, he changed his mind.
他最初的反应是不同意，可是后来他改变了主意。
2. however与but 两者都做“但是，然而”讲，而且都引出并列句。从语义上看，but所表示的是非常明显的对比，转折的意味较however要强。从语序上看，but总是位于所引出的分句之首，而however却可位于句首、句中和句末，同时从标点符号上看，but之后一般不得使用逗号，而however则必须用逗号与句子其它部分分开。例如：
I thought those figures were correct. However, I have recently heard they were not.
我原以为那些数字正确无误，不过我最近听说并不正确。
3. however的其它用法： however可以作连词(conj.)，表示“无论以何种方式，不管怎样”，引导让步状语从句。例如：
You can travel however you like.你可以随心所欲地去旅行。
However I approached the problem, I couldn’t find a solution.
这一问题我不管怎样都无法解决。

but 用来引出微弱的相反的意见，是口语常用词。
Nearly everybody enters for‘The Nicest Garden Competition’each year,but Joe wins every time.
年年几乎大家都报名参加“最佳花园竞赛”，但每次获胜的都是乔。
…but more often than not,the greater part of the journey is spent on narrow,bumpy roads which are crowded with traffic.
但是一路之上，大部分的时间往往会在狭窄、坎坷不平而且车辆拥挤的道路上度过……(more often than not=very frequently[英语惯用法])
It's a pleasant dream but everything depends on"if".
这倒是个非常令人愉快的梦想，但一切都取决于“如果”。
Sally was amused,but I was very embarrassed.
萨莉很开心，但我却很尴尬。
(3) however 比 but 的词义还要弱些，并不直接引出相反的意见，因此常用作插入语
You will be lucky if he lets you go without a ticket.However,this does not always happen.
他如果不给你一个违章通知单就放你走，你就是运气好。不过，这种事也并不总是发生的。
The most surprising thing about it,however,is that it can land anywhere…
不过，关于这架飞机的最使人吃惊的事情是，它可以在任何地方着路……
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简单来说：1.but相对于however比较口语化
2.but必须直接连句子,而however可以直接连句子,也可以有逗号相隔.
3.however可用于议论文中另一观点的开头.
However he did it, it was very clever.
不管他是怎么做的，他做得很聪明
The book is expensive; however, it's worth it.
这本书很贵；却很值
the plan caused not prosperity but ruin.
这个计划带来的不是繁荣而是毁灭 ——

辨析 however和 but
二者都意为“可是，但是”；
but是并列名词，连接两个并列分句，上下二句在总的意义上构成了对比。
I really don't like cheese, but I will try just a little this time.我实在是不喜欢吃奶酪，但是这次我要尝一点。
however不能直接连接两个分句，必须另起新句，并用逗号隔开。
It was raining hard. However, we went out to look for the boy. 雨下得很大，不过我们还是出去寻找那个孩子