There are really only two conjunctions in Japanese that can combine two sentences together into a compound sentence: "but" and "because". However, there are several versions of each with a slightly different level of politeness.
You may remember we already used 「でも」 to mean "but" or "however". While 「でも」 is always used at the beginning of a new sentence, there are two conjunctions that also mean "but" and can be used to combine two sentences together into one compound sentence, similar to English. The two conjunctions are 「けど」 and 「が」. 「けど」 is fairly casual while 「が」 is slightly more formal and polite. (Note that this 「が」 is completely different from the identifier particle we learned in the last section.)
Note: If the first clause ends with a noun or na-adjective without any tense and you're not using 「です」, you must add 「だ」.
If the noun or na-adjective is already conjugated (such as the negative 「じゃない」), you don't need to add 「だ」.
You can combine two sentences with 「から」 or 「ので」 to show a reason and result but it's important to remember that the reason comes first. Therefore, it may be beneficial to remember it as meaning "so" rather than "because" to match the order. 「ので」 is slightly more polite and formal compared to 「から」.
(It's) noisy here so (I) don't like it very much.
Note: Once again, If the first clause ends with a noun or na-adjective without any conjugation (such as the negative 「じゃない」), and you're not using 「です」, you must add 「だ」 for 「から」. However, for 「ので」 you must add 「な」 instead.
Once again, this only applies to nouns and na-adjectives that are not conjugated to another tense.
Similarly, you can attach two sentences with 「のに」 to mean "despite" or "in spite of". Similar to 「ので」, you must attach 「な」 when the first clause ends with a plain noun or na-adjective.
You can leave out either side of the conjunction if it's understood by context.
スミス: ここは、好きじゃない。
Smith: I don't like (it) here.
リー: なんで?
Lee: Why?
スミス: うるさいから。
Smith: Because it's noisy.
If you leave the first part out, you still need to add 「です」, 「だ」, or 「な」 just as if the first sentence was there.
りー: 図書館なのに、ここはいつもうるさいよね。
Lee: Despite the fact (it's) library, (It's) always noisy here, huh?
スミス: だから、あまり好きじゃない。
Smith: That's why I don't like (it) very much.
Other options would be the same as it would be with both sentences.
You can even leave out both parts of the conjunction as seen in the next dialogue.
リー: 最近は、忙しいよ。
Lee: I'm busy lately, you know?
スミス: だから?
Smith: So?
リー: だから、大変だよ!だけど今日は、やっと暇だから、うれしい!
Lee: So (it's) tough, you know! But (I'm) finally free today so (I'm) happy!
スミス: あっ、そう?
Smith: Oh, is that so?