Although it was expensive, but I bought it. これは英語では誤りです。although が「〜だけれど」を、but が「しかし」を担っていて、ひとつのつなぎ目に部品が2つ入っています。英語は1つのつなぎ目に1つだけ。どちらかを捨ててください。同じことが Because …, so … でも起きます。
"Although it was expensive, but I bought it" doesn't work. Although already carries the contrast, and then but carries it a second time — two connectors in one joint. English allows only one. Drop whichever you like, but drop one. The same trap waits in "Because …, so …."
ひとつのつなぎ目に、部品はひとつ
Two Connectors Won't Fit
although はそれ自体が「対比」を伝える部品です。使った時点で仕事は終わっていて、but を足しても意味は強まりません。英語の読み手には、ネジが二重に締めてあるように見えます。直し方は単純で、although を残すか but を残すかを選ぶだけ。どちらを選ぶかで、文の重心が少し動きます。
Although carries the contrast on its own. Once it's there, the job is done, and adding but doesn't make the contrast stronger. To an English reader it looks like the same screw turned twice. The repair is simple: keep although or keep but. Which one you keep shifts where the weight of the sentence lands.
どちらかに寄せるPick one and commit
Although it was expensive, I bought it.
高かったけれど、買った。
although 節は前置き。主眼は「買った」ほうにある。The although clause is background; "I bought it" is the point.
It was expensive, but I bought it.
高かった。でも買った。
2つの事実を対等に並べる。会話ではこちらが自然。Two facts, side by side and equal. This is the natural spoken shape.
It was expensive. I bought it anyway.
高かった。それでも買った。
接続詞を使わず anyway で受ける手もある。You can also drop the connector and let "anyway" do the work.
although に寄せる
対比を前置きに格下げして、後半を目立たせる。書き言葉で好まれる。Demotes the contrast to background and spotlights the second half. Writing likes this.
Although we left early, we still missed the train.
早く出たのに、それでも電車に乗り遅れた。
but に寄せる
2つを対等に並べる。テンポが軽く、会話向き。Sets the two side by side. Lighter rhythm, better for speech.
We left early, but we still missed the train.
早く出た。でも電車には乗り遅れた。
よくある間違い / Common Mistake
×Because I was tired, so I went home early.
疲れていたので、それで早く帰った。(のつもりで)
○Because I was tired, I went home early.
疲れていたので、早く帰った。
although と but とまったく同じ重複です。because が「〜なので」を、so が「だから」を担っていて、部品が2つ。because を残すか、I was tired, so I went home early. のように so を残すか、どちらかにします。理由と結果はひとつのつなぎ目で足ります。
This is the same doubling in different clothes. Because supplies the reason and so supplies the result, so the joint has two parts in it. Keep because, or rewrite as "I was tired, so I went home early" and keep so. One joint handles reason and result together.
even though実際は〜なのに事実であることを強めた although。後ろは文。even if(仮定)とは別物。Although with the fact underlined. A clause follows. Not the same as even if.
..., though.——でもね文末の though は会話の定番。although にこの使い方はない。Sentence-final though is everyday speech. Although never appears here.
even soそれでも接続副詞なので前をピリオドで切る。although の代わりにはならない。A connective adverb, so the sentence before it closes. It can't replace although.
... anyway.それでも／どのみち接続詞を使わずに対比を出す手。会話で頻出。Gets the contrast across with no connector at all. Very common in speech.
まとめ / Summary
although と but を同じ文で重ねない。どちらかを選ぶ。Never run although and but in the same joint. Choose one.
because と so の重複も同じ誤り。Because plus so is the same mistake wearing different clothes.
although に寄せると後半が目立ち、but に寄せると対等になる。Although spotlights the second half; but keeps the two halves level.
文末の though =「でもね」。会話で頻出。Though at the end of a sentence means "still," and speech uses it constantly.