I think so. の so(節のかわりをする so と not)I Think So: One Word for a Whole Clause
代名詞 / Pronouns
"Is it going to rain?" "I think so." の so は、it's going to rain という節をまるごと引き受けています。否定の受け方は動詞で2通りに分かれます: I don't think so.(think 型)と I hope not.(hope 型)。この分かれ方は「否定の繰り上げ」の動詞分類とおおむね重なりますが、両方を取る動詞もあります。
In "Is it going to rain?" "I think so," the so shoulders the whole clause it's going to rain. The negative side splits two ways by verb: I don't think so (the think type) and I hope not (the hope type). The split lines up closely with the negative-raising families, though some verbs accept both.
節を一語に畳む
Folding a Clause into One Word
相手の言ったことを繰り返すのは冗長です。Do you think it's going to rain? に、節を丸ごと言い直して返す人はいません。英語は節を so 一語に畳んで I think so. と返します。この型を取るのは think・hope・suppose・believe・guess・be afraid など、意見や見込みを述べる動詞たち。否定は2通りあります。否定を前の動詞へ繰り上げる think 型は I don't think so.、繰り上げない hope 型は否定を so の席に入れて I hope not.。「来ないと思う」を I don't think … と前で否定するのと同じ力学が、ここでも働いています。ただし完全に一対一ではありません。suppose や expect は I don't suppose so. と I suppose not. のどちらも使える両刀で、はっきり片方に寄るのは think・believe(don't 側)と hope・be afraid(not 側)です。
Nobody answers Do you think it's going to rain? by saying the whole clause again. English folds it into one word: I think so. The pattern belongs to verbs of opinion and expectation: think, hope, suppose, believe, guess, be afraid. The negative goes two ways. Verbs that raise their negative say I don't think so; verbs that don't raise put the negative into so's seat instead: I hope not. The same force that produces I don't think he'll come is at work here. The match isn't one-to-one, though: suppose and expect take either shape — I don't suppose so and I suppose not are both current — while think and believe lean firmly to the don't side and hope and be afraid to the not side.
so と not の席The seats of so and not
"Will theなぜ the なのか【決まった場所・乗り物】the station / the bus のように、話し手と聞き手の生活で「あの一つ」に決まる施設や便を指す。 trains beこの文の組み立て【未来進行】will be ＋ -ing。未来のある時点で進行中であることを言う形。will ＋ 原形より「もう決まっている予定」として自然に響き、相手の予定を尋ねるときにも使う。running?" "I think so."
「電車、動いてるかな」「動いてると思うよ」
so =直前の節の代用。So stands in for the whole clause.
"Is theなぜ the なのか【その場で一つに決まる】話し手と聞き手のいる状況で、どれを指すか一つに決まるから the。日本語なら「（例の）」「うちの」を補って読むと感覚が近い。 meeting still on?" "I'm afraid not."
「会議、まだやる?」「残念だけど、ないみたい」
be afraid は not 型。悪い知らせの確認には I'm afraid so. もある。Be afraid takes not — and I'm afraid so confirms bad news.
If so, please let me know.
もしそうなら、知らせてください。
if so / if not は書き言葉の定型。If so and if not are set written frames.
don't ... so 型(繰り上げる)
think・believe が代表。I don't think so. が標準(suppose・expect は not 型も使える)。Think and believe lead this group: I don't think so is the standard shape. (Suppose and expect take the not shape as well.)
"Is he coming back?" "I don't believe so."
「彼、戻ってくる?」「戻らないと思うよ」
... not 型(繰り上げない)
hope・be afraid・guess。否定は後ろに残す: I hope not.Hope, be afraid, guess: the negative stays behind — I hope not.
"Is it broken?" "I hope not!"
「壊れた?」「そうじゃないといいけど!」
よくある間違い / Common Mistake
×"Do we have to redo it?" "I think not."
「やり直し?」「そうは思わないけど」(ふつうの返事のつもりで)
○"Do we have to redo it?" "I don't think so."
「やり直し?」「そうは思わないけど」
I think not. は文法的には正しい英語ですが、現代の会話では芝居がかった、皮肉めいた響きになります(わざと使うユーモアの定型でもあります)。ふつうに否定するなら I don't think so.。think は否定を前へ繰り上げる型だからです。hope は繰り上げない型なので、逆に I don't hope so. とは言いません。迷ったら: think・believe 系は don't … so、hope・afraid は … not。
I think not is grammatical, but in modern conversation it lands theatrical, even arch — it survives mainly as deliberate humor. The plain negative is I don't think so, because think raises its negative forward. Hope runs the other way and never raises: "I don't hope so" isn't English. When unsure, think and believe take don't … so, while hope and afraid take not.
I think so. / I don't think so.そうだと思う/そうは思わない代用 so の基本の対。会話の最頻出。The core pair, everywhere in conversation.
I hope so. / I hope not.そうだといいな/そうじゃないといいなhope は not 型。期待の向きで使い分ける。Hope's own pair — never raised, just aimed.
If so, ... / If not, ...そうであれば/そうでなければ節を畳んだ条件文。メール・案内文の定番。Folded conditionals, standard in email and notices.
I told you so.だから言ったでしょ。「そうなると言った」の so。勝ち誇りが乗るので使いどころに注意。The so of told-you warnings — gloating built in, handle with care.
まとめ / Summary
so は直前の節をまるごと受ける(I think so.)。So carries the whole previous clause: I think so.
否定は2通り: think 型は I don't think so.、hope 型は I hope not.。Negatives split two ways: I don't think so, but I hope not.
分かれ方は「否定の繰り上げ」とおおむね同じ。suppose・expect は両刀。The split mostly follows the negative-raising families; suppose and expect take either.
I think not. は芝居がかった響き。日常は I don't think so.。I think not sounds theatrical; everyday English says I don't think so.