第11講：助動詞 — 能力・許可・義務・可能性

助動詞（modal verb）は、話者の態度や心理的な視点を表す動詞です。助動詞の後には必ず本動詞の原形（toなし、-sなし）が来ます。

助動詞 完全参照表

助動詞	主な意味	現在/未来の例文	過去の例文
can	能力、非公式な許可	She can speak French.	She could speak French as a child.
could	過去の能力、丁寧な依頼、可能性	Could you help me?	I couldn’t find it.
may	公式な許可、可能性	You may leave now.	(might have)
might	弱い可能性、提案	It might rain later.	She might have forgotten.
must	強い義務（内的）、推論	You must stop at red lights.	He must have been tired.
have to	外的義務、必要性	I have to submit it by Friday.	She had to leave early.
should	アドバイス、勧告、期待	You should see a doctor.	You should have called.
will	未来、自発的意志、予測	I will help you.	(would)
would	丁寧な依頼、仮定、過去の習慣	Would you mind closing the door?	He would walk to school every day.
shall	公式な未来（I/we）、提案	Shall we begin?	—

推論を表す助動詞

助動詞を使って、ある事実に対する確信の度合いを表すことができます：

確信の度合い	現在	過去
100% 確信（肯定）	He must be home.	He must have been home.
約 80% 確信	She should be there by now.	She should have arrived.
約 50% 可能性	It may/might be true.	It may/might have been true.
100% 確信（否定）	That can’t be right.	That can’t have been right.

よくあるミス

1. 助動詞の二重使用: ❌ You might can do it. → ✅ You might be able to do it.
2. 否定形でのmustとhave toの混同: You mustn’t do it（禁止）≠ You don’t have to do it（義務なし）
3. 単発の過去の達成にcouldを使用: ❌ I could finish it in time. → ✅ I was able to finish it in time.

丁寧さのレベル別助動詞

非格式 → 格式	依頼の表現
最も非格式	Can you help me?
やや丁寧	Could you help me?
格式	Would you help me?
非常に格式	Would you mind helping me?

今日の講義のまとめ

• 助動詞の後に本動詞の原形を正しく使える。
• 否定形での must（禁止）と don’t have to（義務なし）の違いを理解する。
• 推論を表す助動詞（must/might/can’t + have + 過去分詞）を使える。