基本の説明Overview
助動詞は、動詞の前に置いて「できる」「〜しなければならない」「〜すべきだ」「〜してもよい」などの意味をつけ加える語です。英語では、事実をそのまま言うだけでなく、能力・義務・助言・許可といった話し手の気持ちや判断を、助動詞で動詞にのせて伝えます。A modal verb goes in front of the main verb and adds a meaning such as ability, obligation, advice or permission. Modals let the speaker add a feeling or judgment — not just state a fact.
助動詞のあとの動詞は、主語が何であっても必ず原形です。He can plays のように三単現の s をつけたり、過去形にしたりしません。また、助動詞自体にも s はつかず、疑問文では助動詞を主語の前に出し、否定文では助動詞のあとに not を置きます。do / does / did は使いません。The verb after a modal is always in the base form, whatever the subject. Don't add the third-person -s (not “He can plays”), and don't put it in the past tense. The modal itself takes no -s either. For a question, move the modal in front of the subject; for a negative, put not after it. Do not use do / does / did.
| 助動詞Modal | 主な意味Main meaning | 否定の形と意味Negative form and meaning |
|---|---|---|
| can | 〜できる（能力）／〜してもよい（許可）can do (ability) / may do (permission) | cannot (can't) — 〜できないcannot do |
| may | 〜してもよい（許可）／〜かもしれないmay do (permission) / might be | may not — 〜してはいけない／〜ではないかもしれないmust not do / might not be |
| must | 〜しなければならない（強い義務）must do (strong obligation) | must not (mustn't) — 〜してはいけない（禁止）must not do (prohibition) |
| have to | 〜しなければならない（主語が三単現なら has to）have to do (has to with he / she / it) | don't have to — 〜する必要はないneed not do |
| should | 〜すべきだ、〜したほうがいい（助言）should do (advice) | should not (shouldn't) — 〜しないほうがいいshould not do |
must と have to はどちらも「〜しなければならない」ですが、否定になると意味が大きく変わります。must not は「〜してはいけない」という禁止、don't have to は「〜しなくてもよい」という不必要です。must には過去形がないので、過去の義務は had to で表します。Must and have to both mean “have to do,” but their negatives are very different: must not is a prohibition (“not allowed to”), while don't have to means there is no need. Must has no past form, so past obligation is expressed with had to.
相手に何かをお願いしたり、申し出たり、さそったりするときにも助動詞を使います。Can you ~? / Could you ~?（〜してくれませんか。could のほうがていねい）、Shall I ~?（〜しましょうか）、Shall we ~?（〜しませんか）がよく使われます。Modals are also used to ask, offer and suggest. Common patterns are Can you ~? / Could you ~? (a request; could is more polite), Shall I ~? (an offer) and Shall we ~? (a suggestion).
例文で確認Example sentences
① can（できる・してもよい）1. Can (ability and permission)
② may（許可・推量）2. May (permission and possibility)
③ must / have to（義務・禁止・不必要）3. Must and have to (obligation, prohibition, no need)
④ should（助言）4. Should (advice)
⑤ お願い・申し出・提案5. Requests, offers and suggestions
確認クイズQuick check
空所に適する語を入れましょう。（ ）の語は、必要なら形を変えます。Fill in each blank with a suitable word. Change the word in parentheses if necessary.
- Tom can ___ (play) the guitar.
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play — 助動詞のあとは原形。plays にしない。A base form follows a modal; not plays. - You ___ not swim in this river. It is dangerous. （禁止）
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must — must not で「〜してはいけない」という禁止。Must not expresses prohibition. - My mother ___ to work on Sundays. （have to の形で）
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has — 主語が三人称単数なので has to。The subject is third-person singular, so has to. - ___ we go to the movies tomorrow? （〜しませんか）
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Shall — Shall we ~? で「〜しませんか」とさそう。Shall we ~? is a suggestion: “Why don't we…?” - You look tired. You ___ go to bed early. （〜したほうがいい）
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should — 助言を表すのは should。Should expresses advice. - You don't ___ wear a coat today. It is warm. （空所に2語）
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have to — don't have to で「〜する必要はない」。Don't have to means “there is no need to.”
おさえておきたいポイントKey points
- 助動詞のあとの動詞は、主語に関係なく必ず原形（三単現の s もつけない）The verb after a modal is always in the base form, whatever the subject (no third-person -s)
- can = できる、may = してもよい、must = しなければならない、should = すべきだcan = ability, may = permission, must = obligation, should = advice
- must not は「〜してはいけない（禁止）」、don't have to は「〜しなくてもよい（不必要）」で、意味がちがうMust not means “not allowed to,” while don't have to means “no need to” — they are different
- 疑問文は助動詞を主語の前に、否定文は助動詞のあとに not。do / does / did は使わないFor questions, put the modal first; for negatives, put not after it. Do not use do / does / did
- must に過去形はないので、過去の義務は had to。助動詞を2つ重ねず、can の代わりに be able to を使うこともあるMust has no past form, so use had to for past obligation. Don't stack two modals; be able to can stand in for can
- Can you ~? / Could you ~?（お願い）、Shall I ~?（申し出）、Shall we ~?（提案）はセットで覚えるLearn these set patterns: Can / Could you ~? (request), Shall I ~? (offer), Shall we ~? (suggestion)