Tom works very hard at his lessons.He did not get the first in the exam.

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,
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.