but 是连词，连接两个并列句，中间用逗号隔开。表示转折
如： I wanted to buy a book, but I don't have enough money.
though 是副词，但在句中的成分相当于连词。引导从句，不能与but 连用，可以放句首或者句中。意思是尽管，though 和although不可以与but连用，解释如下:
1.(al)though 是从属连词连接一个从句，but是并列连词连接一个并列句，一个句子要么是主从句要么是并列句，所以两者不可能出现在同一个句子中，只能单个使用：
Although it was expensive, we decided to buy the computer.=The computer was expensive,but we decided to buy it.
2. however语气比but强，但由于同样的理由不能与(al)though 出现在同一个句子中。
如：Though it is so hot I have to go outside.
I have to go outside though it is so hot.
however 也是副词，剧中成分相当于连词。可以放在句首或句中，后面一定要用逗号。表转折。放句首时，与前面的句子是转折的连接。放句首时，前后用逗号隔开
如：I wanted to buy a book, however, i do not have enough money.
区别：①although和though同义，在一般情况下，可以互换使用。只是though较普遍，常用于非正式的口语或书面语中，而although则较为正式。另外although语气比though重，常用以强调让步概念。
e.g. Though/Although my car is very old, I don't want to buy a new one.
虽然我的汽车很旧，但我不想买一辆新的。
He insisted on doing it although I warned him not to.
虽然我警告他不要做，但他仍然坚持要做。
②even能和though组合表示强调，但even不能和although组合。因此，不能说even although,在这里even though=even if(no matter if; though即使……也……)
e.g. Even if/Even though we could afford it, we wouldn't go abroad for our vocation.
即使我们付得起这笔钱，也不出国度假。
③当让步状语从句指某种假设情况时，通常用though,而不用although。
e.g. Though all the world were against me, I should still hold to my opinion.
就算全世界都反对我，我还是坚持我的立场。
Though everybody deserts you, I will not.
即使所有人都抛弃你，我也不会的。
④though可以独立用作副词，常放在句末，意为all the same (还是；仍然)或however(可是；然而)，而although无此用法。
e.g. She didn't tell me what she had done, but I know it though.
她没有告诉我她所做的事情，但我还是知道了。
It's hard work. I enjoy it, though.
工作很苦，可是我喜欢。
⑤though引导让步状语从句时可以倒装，而although从句则不能。
e.g. Bravely though they fought, they had no chance of winning.
虽然他们打得很勇敢，但还是没有机会获胜。
相同点：①在although和though引导的让步状语从句中，主句里不可用but,若要强调前后两部分的对比意义，可在主句前加yet或still。
e.g. Although/Though he has a lot of money, yet/still he is unhappy.
他虽然有很多钱，但并不幸福。
②although和though引导让步状语从句时，常采用省略形式。
e.g. Although tired (=Although he was tired), he kept on working.
虽然很疲劳，但他继续工作。
The problem, though complicated(=though it is complicated), can be solved without much difficulty.
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