だけど

1. 1Attached to a noun or な-adjective, it connects a statement to a contrasting one ('X, but Y').Ex. 1
2. 2It links a circumstance to a result that goes against what one might expect ('although X, Y').Ex. 2
3. 3Used at the start of a sentence as a conjunction meaning 'But…' in response to what came before.

だけど is typical of everyday conversation and casual writing such as messages to friends. It is だ plus けど, so it appears where the preceding element is a noun or な-adjective, or at the start of a sentence. It is less formal than けれども, and in formal writing しかし or ですが are preferred.

• けど / けれどもN5「けど／けれども」attaches directly to verbs and い-adjectives, as in 「高いけど」; after nouns and な-adjectives use 「だけど」 or 「ですけど」. 「けれども」 is more formal, so it can replace 「だけど」 when speaking to a superior.
• でもN5Mid-sentence 「でも」 means 'things like...', 'even if...', or 'no matter...', and is not a contrastive conjunction: 「雨だけど」 means 'although it rains,' while 「雨でも」 means 'even if it rains.' At the start of a sentence meaning 'but,' it is close to 「だけど」 and the two are basically interchangeable.
• それなのにN2「それなのに」 only connects two sentences at the start of the second one, similar to sentence-initial 「だけど」, but it specifically indicates an outcome that defies common sense, carrying a tone of surprise, blame, and regret; it cannot be used for simple contrast, and the following clause cannot be a command, request, or expression of volition. Its register is neutral.
• ところがN3「ところが」 emphasizes an unexpected outcome that goes against one's wishes; the following clause is usually a fact that has already happened, and commands, requests, and the like cannot follow. Its register is neutral, and it can also attach to the ta-form of verbs. 「だけど」 is a general contrastive conjunction and more colloquial. The two are interchangeable when introducing an unexpected fact at the start of a sentence.
• のにN4「のに」 emphasizes an outcome contrary to expectations, often with dissatisfaction or regret, and the following clause cannot be a command or request; 「だけど」 is merely a general contrastive conjunction. 「のに」 attaches directly to verbs and い-adjectives, takes the form 「なのに」 after nouns and な-adjectives, and can also appear at the end of a sentence to express regret.

けど/けれども attach directly to verbs and い-adjectives, with けれども being more formal. Nouns and な-adjectives need だ before them, which gives exactly だけど (静かだけど, not 静かけど), and でも is a casual sentence-initial 'but' close to sentence-initial だけど. それなのに and のに carry a stronger sense of frustration or dissatisfaction at the contradiction, while ところが marks an unexpected turn of events in narration rather than a simple contrast.