Modal Verbs (助動詞) - 完全ガイド

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Can / Could

Can — 能力、許可、可能性

1. I can speak three languages. (能力: 3か国語話せる)
2. Can I use your phone? (許可: 使っていい?)
3. It can get very cold here in winter. (可能性: 冬はとても寒くなりうる)
4. You can't park here. (禁止)
5. Can you help me? (依頼)
6. That can't be true! (不可能: ありえない!)
7. You can never be too careful. (いくら気をつけても足りない)

Could — can の過去、丁寧な依頼、弱い可能性

1. I could swim when I was 5. (過去の能力: 5歳で泳げた)
2. Could you please close the door? (丁寧な依頼)
3. It could rain later. (可能性: 雨が降るかもしれない — may より弱い)
4. She could be at home. (推量: 家にいるかもしれない)
5. I couldn't understand what he said. (過去: 理解できなかった)
6. You could try calling her. (提案: 電話してみたら?)
7. I could have helped you. (過去の仮定: 手伝えたのに)

Can vs Could — ニュアンス

• Can you help? (カジュアル)
• Could you help? (より丁寧)
• He can be rude sometimes. (時々失礼なことがある — 一般的可能性)
• He could be at the office. (オフィスにいるかもしれない — 推量)

can't have / couldn't have — 過去の不可能

1. She can't have said that. (彼女がそんなこと言ったはずがない)
2. He couldn't have done it alone. (一人でやれたはずがない)

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May / Might

May — 許可(フォーマル)、可能性(50%くらい)

1. May I come in? (入ってもよろしいですか? — 非常にフォーマル)
2. You may leave now. (出ていってよろしい — 上から許可)
3. It may rain tomorrow. (明日雨が降るかもしれない — 五分五分)
4. She may be busy. (忙しいかもしれない)
5. He may not come. (来ないかもしれない)

Might — may より弱い可能性(30%くらい)

1. It might snow tonight. (今夜雪が降るかもしれない — 可能性低め)
2. She might be interested. (興味があるかもしれない)
3. I might go to the gym later. (後でジムに行くかもしれない — 未定)
4. You might want to check this. (これ確認したほうがいいかも — 控えめな提案)
5. He might have forgotten. (忘れたのかもしれない)

may have / might have — 過去の推量

1. She may have missed the train. (電車に乗り遅れたのかもしれない)
2. He might have been joking. (冗談だったのかもしれない)
3. They may have already left. (もう出発したかもしれない)

May vs Might — 使い分け

• may = 可能性がそこそこある (50%)
• might = 可能性が低い (30%) / より仮定的
• 実際にはネイティブもそこまで厳密に区別しない
• 仮定法では might を使う: If I tried harder, I might succeed.

忘れがちなポイント

• May I? はとてもフォーマル。カジュアルなら Can I? を使う
• "You may" は教師や上司が使う表現。友人同士では不自然
• might as well = 〜してもいいかも / どうせなら
• We might as well stay home. (どうせなら家にいよう)

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Must / Have to

Must — 義務(話者の判断)、強い推量

1. You must wear a seatbelt. (シートベルトをしなければならない — ルール)
2. I must finish this report today. (今日中にこの報告書を終えなければ — 自分の判断)
3. You must be tired after such a long flight. (長いフライトの後で疲れているに違いない — 推量)
4. She must be at least 40. (40歳以上に違いない)
5. You mustn't smoke here. (ここで喫煙してはいけない — 禁止)
6. This must be a mistake. (これは間違いに違いない)

Have to — 義務(外的要因)

1. I have to wake up at 6 AM. (6時に起きなければならない — 仕事のため)
2. You have to pay taxes. (税金を払わなければならない)
3. She has to work on Saturdays. (土曜日に働かなければならない)
4. Do I have to go? (行かなければならない?)
5. You don't have to come if you don't want to. (来なくてもいい — 義務がない)

Must vs Have to — 超重要な違い

Must	Have to
話者の判断・内的義務	外的ルール・義務
I must study. (自分で思う)	I have to study. (テストがあるから)
mustn't = 禁止	don't have to = 不要

mustn't vs don't have to — 絶対間違えない!

• You mustn't touch that. = 触ってはいけない! (禁止)
• You don't have to touch that. = 触らなくてもいい (不要 — 触ってもOK)

must have — 過去の確信的推量

1. She must have forgotten. (忘れたに違いない)
2. He must have been very happy. (とても嬉しかったに違いない)
3. They must have left already. (もう出発したに違いない)

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Should / Ought to

Should — アドバイス、期待、義務(弱い)

1. You should exercise more. (もっと運動すべきだ — アドバイス)
2. You should see a doctor. (医者に診てもらうべきだ)
3. He should be here by now. (もうここにいるはずだ — 期待)
4. You shouldn't eat so much junk food. (ジャンクフードをそんなに食べるべきでない)
5. Should I bring anything? (何か持っていくべき?)
6. You should apologize. (謝るべきだ)

Ought to — should とほぼ同じ(ややフォーマル)

1. You ought to be more careful. (もっと気をつけるべきだ)
2. She ought to know better. (もっとわかるべきだ)
3. He ought to apologize. (謝るべきだ)

should have — 過去のアドバイス(後悔)

1. I should have studied harder. (もっと勉強すべきだった — 実際はしなかった)
2. You shouldn't have said that. (あんなこと言うべきではなかった)
3. We should have left earlier. (もっと早く出発すべきだった)
4. He should have told me. (私に言うべきだった)

忘れがちなポイント

• should have + pp = 過去の後悔/批判 (〜すべきだったのに)
• shouldn't have + pp = 過去の批判 (〜すべきでなかったのに)
• It should be fine. = 大丈夫なはずだ (期待)

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Will / Would

Will — 未来、意志、習慣、確信

1. I will call you tomorrow. (明日電話する)
2. I will never give up. (絶対諦めない — 意志)
3. He will often sit there for hours. (よくそこに何時間も座っている — 習慣)
4. That will be John at the door. (ドアにいるのはジョンだろう — 確信)
5. Will you help me? (手伝ってくれる?)
6. She won't listen to anyone. (誰の言うことも聞かない — 拒絶)

Would — 過去の習慣、丁寧、仮定

1. When I was young, I would play in the park every day. (昔は毎日公園で遊んだものだ)
2. Would you like some coffee? (コーヒーはいかがですか?)
3. Would you mind closing the door? (ドアを閉めていただけますか?)
4. I would love to go. (ぜひ行きたい)
5. If I were rich, I would travel the world. (仮定法)
6. She would always bring homemade cookies. (いつもクッキーを持ってきたものだ)

would have — 過去の仮定の結果

1. I would have helped if you had asked. (頼まれれば手伝っていたのに)
2. She would have come if she had known. (知っていたら来ていたのに)

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Shall

フォーマル/イギリス英語的。アメリカではほぼ使わない。

1. Shall I open the window? (窓を開けましょうか? — 申し出)
2. Shall we dance? (踊りましょうか? — 提案)
3. Shall we go? (行きましょうか?)
4. You shall not pass! (通ってはならぬ! — 文学的/映画的)

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Need

Modal verb (フォーマル)

1. You needn't worry. (心配する必要はない)
2. Need I say more? (これ以上言う必要がある?)
3. She needn't have brought a gift. (お土産を持ってくる必要はなかったのに — 実際は持ってきた)

Regular verb (一般的)

1. You don't need to worry. (= needn't)
2. Do I need to bring anything? (何か持っていく必要ある?)

needn't have vs didn't need to

• She needn't have bought so much food. (たくさん買う必要なかったのに — 買ってしまった)
• She didn't need to buy food. (食べ物を買う必要がなかった — 買わなかった可能性も)

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Modal Verbs for Deduction (推量)

Certainty	Positive	Negative
100%	It is true.	It isn't true.
95%	It must be true.	It can't be true.
50%	It may be true.	It may not be true.
30%	It might/could be true.	It might not be true.

Past deduction

Certainty	Positive	Negative
95%	He must have left.	He can't have left.
50%	He may have left.	He may not have left.
30%	He might have left.	He might not have left.

Examples

1. She's not answering. She must be busy. (95% — 忙しいに違いない)
2. He can't be 60! He looks so young! (ありえない!)
3. She may be at the gym. (50% — ジムにいるかも)
4. He might have gotten lost. (30% — 迷ったかもしれない)
5. It could be a virus. (30% — ウイルスかもしれない)
6. They must have heard the news by now. (もう聞いたに違いない)

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Common Mistakes by Japanese Speakers

1. ✗ You must to go. → ✓ You must go. (modal + 原形、toは不要)
2. ✗ I can to swim. → ✓ I can swim.
3. ✗ He can speaks English. → ✓ He can speak English. (三単現のs不要)
4. ✗ I must not go = 行かなくてもいい → ✓ must not = 禁止! / don't have to = 不要
5. ✗ She should studied harder. → ✓ She should have studied harder.
6. ✗ You don't must go. → ✓ You mustn't go. (禁止) / You don't have to go. (不要)

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Practice Exercises

Choose the correct modal:

1. You ___ be tired after working 12 hours. (must — 確信的推量)
2. ___ I borrow your pen? (Can/Could/May — 許可)
3. You ___ smoke here. It's forbidden. (mustn't — 禁止)
4. You ___ bring food. There will be plenty. (don't have to — 不要)
5. She ___ have left already. Her car is gone. (must — 確信的推量)
6. I ___ have told her the truth. (should — 過去の後悔)
7. He ___ swim when he was three. (could — 過去の能力)
8. It ___ rain this afternoon. Take an umbrella just in case. (might/could — 弱い可能性)