基本の説明Overview
仮定法は、現在や過去の事実とは違うこと、あるいは実現する可能性がとても低いことを、「もし〜なら、…なのに」と表す形です。「もし明日雨なら」のように実際に起こりうることを言うふつうの条件文とは違い、頭の中で想像した「ありえない（ありそうにない）話」を言うときに使います。The subjunctive talks about things that are not true now or in the past, or that are very unlikely: “If X were true, Y would happen.” It differs from an ordinary conditional like “if it rains tomorrow”, which describes something that could really happen. It is for imagined situations.
仮定法の大きな特徴は、動詞の形を一歩だけ過去にずらすことです。ここでの過去形は「昔のこと」ではなく、「現実から離れている」ことを示す印だと考えるとわかりやすくなります。現在の事実に反することは過去形（仮定法過去）、過去の事実に反することは過去完了形（仮定法過去完了）で表します。The key feature is shifting the verb one step back. The past form here does not mean “long ago”; think of it as a sign that the situation is far from reality. Use the past form (past subjunctive) for something contrary to the present, and the past perfect (past perfect subjunctive) for something contrary to the past.
| 種類Type | if 節If-clause | 主節Main clause | 意味Meaning |
|---|---|---|---|
| 仮定法過去Past subjunctive | 過去形（be動詞は were）past form (be → were) | would / could / might + 動詞の原形would / could / might + base verb | 現在の事実に反するContrary to the present |
| 仮定法過去完了Past perfect subjunctive | had + 過去分詞had + past participle | would / could / might + have + 過去分詞would / could / might + have + past participle | 過去の事実に反するContrary to the past |
仮定法過去：If + 主語 + 動詞の過去形 〜, 主語 + would[could] + 動詞の原形 …. ／ 仮定法過去完了：If + 主語 + had + 過去分詞 〜, 主語 + would[could] + have + 過去分詞 ….Past subjunctive: If + subject + past-tense verb ~, subject + would[could] + base verb …. / Past perfect subjunctive: If + subject + had + past participle ~, subject + would[could] + have + past participle ….if 節の be動詞は、主語が I や he でもふつう were を使う（会話では was も使われる）。would は「〜だろう」、could は「〜できるだろう」の意味。In the if-clause the be-verb is normally were, even after I or he (was is also heard in conversation). Would means “would”, and could means “would be able to”.
仮定法は if 節だけではありません。I wish 〜（〜だったらいいのに）、as if 〜（まるで〜のように）、Without 〜 / If it were not for 〜（〜がなければ）なども、同じように動詞の形をずらして使います。The subjunctive is not limited to if-clauses. I wish ~ (“if only ~”), as if ~ (“as though ~”) and Without ~ / If it were not for ~ (“if there were no ~”) shift the verb form in the same way.
例文で確認Example sentences
① ふつうの条件文との違い1. Compared with an ordinary conditional
If it rains tomorrow, we will stay home.
もし明日雨が降ったら、私たちは家にいます。
雨が降る可能性は十分ある。事実をそのまま言うふつうの条件文なので、仮定法ではない。Rain is a real possibility, so this is an ordinary conditional, not the subjunctive.
If I am free tomorrow, I will help you.
もし明日ひまなら、手伝います。
ひまかもしれないという、実際にありうる話。It may really turn out that I am free.
If I were free now, I would help you.
今ひまなら手伝うのに（実際はひまではない）。
今の事実に反するので、過去形 were と would を使う。This is contrary to the present fact, so it uses the past form were and would.
② 仮定法過去（現在の事実に反する）2. Past subjunctive (contrary to the present)
If I were you, I would tell her the truth.
もし私があなたなら、彼女に真実を話すのに。
実際には「私」はあなたではない。If I were you は、助言をするときの定番表現。In reality I am not you. If I were you is a standard way to give advice.
If it were sunny today, we could go on a picnic.
今日が晴れていたら、ピクニックに行けるのに。
実際には晴れていない。In fact it is not sunny.
If I had more time, I would read more books.
もっと時間があれば、もっと本を読むのに。
if 節は過去形 had。実際には時間がない。The if-clause uses the past form had. In fact I do not have time.
If I knew her phone number, I would call her.
彼女の電話番号を知っていれば、電話をかけるのに。
If I were a bird, I could fly to you.
もし私が鳥なら、あなたのところへ飛んでいけるのに。
③ 仮定法過去完了（過去の事実に反する）3. Past perfect subjunctive (contrary to the past)
If I had studied harder, I could have passed the exam.
もっと一生懸命勉強していたら、試験に合格できたのに。
実際には勉強しなかったし、合格もできなかった。In fact I did not study hard, and I did not pass.
If it had not rained yesterday, we would have gone to the park.
昨日雨が降らなかったら、私たちは公園に行ったのに。
had not rained は hadn't rained と短くしてもよい。Had not rained can be shortened to hadn't rained.
If Emi had left home earlier, she would not have missed the bus.
エミがもっと早く家を出ていたら、バスに乗り遅れなかったのに。
If you had told me, I would have helped you.
言ってくれていたら、手伝ったのに。
If I had known about the party, I would have come.
パーティーのことを知っていたら、行ったのに。
④ I wish 〜（〜だったらいいのに）4. I wish ~ (“if only ~”)
I wish I could fly.
空を飛べたらいいのに。
現在の実現しにくい願い。wish のあとは過去形（could / were など）になる。A wish about the present that is unlikely to come true. After wish, use a past form (could, were, etc.).
I wish I had a bigger room.
もっと大きな部屋があればいいのに。
I wish I were taller.
もっと背が高ければいいのに。
I wish I had studied harder.
もっと一生懸命勉強しておけばよかったのに。
過去のことへの後悔は had + 過去分詞。For regret about the past, use had + past participle.
⑤ as if 〜（まるで〜のように）5. as if ~ (“as though ~”)
He talks as if he were a doctor.
彼はまるで医者であるかのように話します。
実際には医者ではない。In fact he is not a doctor.
He looks as if he had not slept at all.
彼はまったく眠っていないかのように見えます。
「見える」のは今、眠っていないのはそれより前なので had + 過去分詞。He looks that way now, but the lack of sleep came earlier, so had + past participle.
⑥ Without / If it were not for 〜（〜がなければ）6. Without / If it were not for ~ (“if there were no ~”)
Without water, we could not live.
水がなければ、私たちは生きていけません。
Without 〜 は If it were not for 〜 とほぼ同じ意味。Without ~ means almost the same as If it were not for ~.
If it were not for your help, I could not finish this job.
あなたの助けがなければ、この仕事を終えられないでしょう。
Without your advice, I would have failed.
あなたのアドバイスがなかったら、私は失敗していたでしょう。
過去のことなので would + have + 過去分詞。This is about the past, so would + have + past participle.
確認クイズQuick check
（ ）内の語を適切な形に直して、空所に入れましょう。Put the word in parentheses into the right form and fill in the blank.
- If I ___ (be) you, I would study harder.
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were — 仮定法過去。be動詞は主語に関係なく were。Past subjunctive: the be-verb is were regardless of the subject. - If she ___ (have) a car, she could drive us there.
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had — 現在の事実に反する仮定なので過去形 had。This is contrary to the present, so the past form had. - If it ___ (rain) yesterday, we would have stayed home.
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had rained — 主節が would have stayed なので、過去の事実に反する仮定。if 節は had + 過去分詞。The main clause is would have stayed, so this is contrary to the past. The if-clause takes had + past participle. - If I had known, I would ___ (tell) you.
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have told — if 節が had known なので、主節は would + have + 過去分詞。The if-clause is had known, so the main clause is would + have + past participle. - I wish I ___ (can) swim well.
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could — wish のあとは過去形。can は could になる。After wish, use a past form: can becomes could. - He talks as if he ___ (know) everything.
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knew — as if のあとも過去形（実際には知らない）。After as if the verb is also in a past form (in fact he does not know).
おさえておきたいポイントKey points
- 仮定法は「事実と違うこと」「実現しにくいこと」を表す。動詞の形を一歩だけ過去にずらすThe subjunctive expresses things contrary to fact or unlikely. The verb form is shifted one step back
- 現在の事実に反する仮定：If + 過去形 〜, would[could] + 動詞の原形Contrary to the present: If + past form ~, would[could] + base verb
- 過去の事実に反する仮定：If + had + 過去分詞 〜, would[could] + have + 過去分詞Contrary to the past: If + had + past participle ~, would[could] + have + past participle
- if 節の be動詞は、主語が I や he でもふつう were を使うThe be-verb in the if-clause is normally were, even with I or he
- I wish 〜 / as if 〜 も、同じ考え方で動詞の形をずらす。過去のことなら had + 過去分詞I wish ~ and as if ~ shift the verb in the same way; use had + past participle for the past
- 実際に起こりうることは、仮定法ではなく If it rains, 〜 will … のようなふつうの条件文を使うFor things that could really happen, use an ordinary conditional such as If it rains, ~ will …, not the subjunctive
よくある間違いCommon mistake："If I am you, I would tell her." ✗ → "If I were you, I would tell her." ✓ あなたになることは現実にはありえないので、were を使います。"If I am you, I would tell her." ✗ → "If I were you, I would tell her." ✓ Being “you” is impossible in reality, so use were.
よくある間違いCommon mistake："If I had known, I would tell you." ✗ → "If I had known, I would have told you." ✓ if 節が had + 過去分詞なら、主節は would have + 過去分詞にそろえます。"If I had known, I would tell you." ✗ → "If I had known, I would have told you." ✓ When the if-clause is had + past participle, the main clause must be would have + past participle.
よくある間違いCommon mistake："If I would have time, I could go." ✗ → "If I had time, I could go." ✓ if 節の中には、ふつう would を入れません。"If I would have time, I could go." ✗ → "If I had time, I could go." ✓ Normally would does not go inside the if-clause.