however はコンマでつなげない(接続副詞)However Can't Hold Two Sentences Together
接続詞 / Conjunctions
It was raining, however we went out. これは誤りです。however は接続詞ではなく副詞。2つの文をコンマだけでつなぐ力がありません。前をピリオドかセミコロンで切るか、接続詞の but を使ってください。however は「つなぐ」部品ではなく、独立した文の頭に立って向きを変える部品です。
"It was raining, however we went out" is an error. However is an adverb, not a conjunction, so a comma alone can't make it hold two sentences together. Close the first sentence with a full stop or a semicolon, or reach for the real conjunction, but. However doesn't join things — it stands at the head of its own sentence and turns the direction.
However, therefore, moreover, and nevertheless are grouped under the label connective adverbs. The label says connective, but grammatically they behave as adverbs, which is why they can't join two sentences. Three shapes are legal: a full stop, a semicolon, or a real conjunction in their place. Those three cover everything.
合法な3つの形The three legal shapes
It wasこの文の組み立て【過去進行】過去のある時点で進行中だったことを言う形。背景を述べておいて、そこへ別の出来事が割り込む形でよく使う。raining. However, we went out.
雨が降っていた。しかし出かけた。
ピリオドで切る。いちばん無難で、どんな文体でも使える。A full stop. The safest option, and it works in any register.
It wasこの文の組み立て【過去進行】過去のある時点で進行中だったことを言う形。背景を述べておいて、そこへ別の出来事が割り込む形でよく使う。raining; however, we went out.
雨が降っていた。しかし出かけた。
セミコロン。2文の結びつきが強いことを示す。書き言葉寄り。A semicolon signals a tighter link between the halves. Leans written.
It wasこの文の組み立て【過去進行】過去のある時点で進行中だったことを言う形。背景を述べておいて、そこへ別の出来事が割り込む形でよく使う。raining, but we went out.
雨が降っていたが、出かけた。
接続詞に置き換える。but はコンマでつなげる。Swap in a conjunction. But is allowed to work with just a comma.
however(副詞)
文中の位置が自由。ただし前の文は必ず終わっている。Free to move around inside its sentence, as long as the previous one has ended.
We went out, however.
でも、出かけたのだ。
We, however, went out.
私たちのほうは、しかし、出かけた。
but(接続詞)
位置は固定で、2文の境目にしか置けない。かわりにコンマでつなげる。Locked to the seam between the two halves — but it may join them with a comma.
We wanted to stay, but we had to leave.
いたかったが、帰らねばならなかった。
よくある間違い / Common Mistake
×The report was late, therefore the meeting was postponed.
報告が遅れたので、会議は延期された。(のつもりで)
○The report was late, so the meeting was postponed.
報告が遅れたので、会議は延期された。
therefore も however と同じ副詞なので、コンマだけで2文をつなぐと誤りになります(comma splice)。直し方は3つ。so に替える、ピリオドで切って Therefore, the meeting was postponed. とする、セミコロンで ; therefore, とする。日常の文章では so がいちばん自然で、therefore は論文や報告書のような硬い文体で選ばれます。
Therefore is an adverb just like however, so joining two sentences with a comma produces a comma splice. Three repairs: swap in so, break it with a full stop and write "Therefore, the meeting was postponed," or use a semicolon before it. In everyday writing so reads most naturally; therefore belongs to papers, reports, and other formal registers.
nevertheless / nonethelessそれにもかかわらずhowever の硬い版。使い方は同じで、前をピリオドかセミコロンで切る。The formal cousin of however. Same rules: full stop or semicolon in front.
as a resultその結果therefore のやわらかい版。会話でも書き言葉でも使える。A gentler therefore, at home in both speech and writing.
on the other handその一方で2つの側面を並べる対比。片方を否定するわけではない。Lays two sides next to each other. It doesn't reject either one.
on the contraryとんでもない、むしろ逆で直前の内容を打ち消す。対比ではなく否定なので on the other hand と入れ替えられない。Rejects what was just said. That's denial, not contrast, so it can't swap with on the other hand.
That being said, ...とはいえ、however のくだけた代役。前をピリオドで切る点は同じ。A casual stand-in for however. The full stop in front still applies.
まとめ / Summary
however / therefore は副詞。コンマだけで2文をつなげない。However and therefore are adverbs. A comma alone won't join two sentences.
合法な形はピリオド、セミコロン、接続詞への置き換えの3つ。Three legal shapes: full stop, semicolon, or a real conjunction instead.
however は文中を動けるが、but は境目から動けない。However roams inside its sentence; but is fixed at the seam.
後ろに形容詞が続く however は「どんなに〜でも」の別語。However followed by an adjective is the other word, meaning no matter how.