The conjunction 然而 is mostly used in written language. It is followed either by an unexpected outcome or by a contradictory statement.
It has the meanings “however”, “but” and “yet".
I don't eat much, but I'm a size 18
wǒ chī de bú tài duō, rán'ér wǒ shì ge chuān shíbā hào de rén
He has a good job, but he never has enough money
tā yǒu hěn hǎo de yí fèn gōngzuò, rán'ér tā cónglái bú gòu qián
The conjunction 但是
但是 is used in both formal language and written language. It indicates a high degree of contrast between two parts of a sentence.
It is used to emphasize a clause following 但是 in a sentence. 但是 also emphasizes confidence in what has been said.
但是 has the meanings “but”, “however”.
但是 can be shortened to 但.
I really want to go, but I'm too busy
wǒ hěn xiǎng qù, dànshì wǒ tài máng le
I knocked on the door, but nobody answered
wǒ qiāo le mén, dànshì méiyǒu rén yìng mén
The conjunction 可是
可是 is more informal than 但是 and is often used in everyday language.
Note that compared to 可是, 但是 indicates a higher degree of contrast between two parts of a sentence.
The situation following 可是 is usually unfavorable to the speaker.
It has the meanings “but”, “however” and “indeed”.
可是 can be shortened to 可.
I like him, but he doesn't like me
wǒ xǐhuan tā, kěshì tā bù xǐhuan wǒ
She wants to be an actress, but her parents are against it
tā xiǎng dāng yǎnyuán, kěshì tā fùmǔ bù tóngyì
The conjunctions 但是 and 可是 are used almost interchangeably.
The conjunction 不过
不过 indicates a mild contrast and is most commonly used in spoken language.
It is often used to complete a sentence and usually doesn't have a negative connotation.
It has the meanings “only”, “just”, “but” and “nevertheless”.
I like this dress, but it's a bit expensive
zhè jiàn yīfu wǒ hěn xǐhuan, búguò yǒudiǎn guì
It's confidential information, but I know you can be trusted
zhè shì jīmì, búguò wǒ zhīdào nǐ kàodezhù
It is often used to emphasize a clause preceding 不过 in a sentence.