英語の助動詞完全ガイド：全用法を例文で学ぶ
助動詞は短くても強力です。たった一語で、文章は事実から可能性・丁寧な依頼・強い義務へと変わります。このガイドでは、主要な助動詞を機能別にまとめています。以下から検索またはカテゴリで絞り込んでください。
48 items · 6 topics
"I can swim"
I know how to swim (present ability)
"I can swim across the lake without stopping."
"She could read at age four"
She had the ability to read in the past
"My sister could read at age four — she was gifted."
"He can't drive"
He does not have the skill to drive
"He can't drive, so he takes the bus everywhere."
"Could you run faster?"
Were you able to run faster (in the past)?
"Could you run faster when you were younger?"
"I can't believe it"
I am unable to accept or process this
"I can't believe she finished the marathon in under three hours."
"We could do better"
We have the potential to improve
"We could do better if we had more time to prepare."
"Can you hear me?"
Are you able to hear what I am saying?
"Can you hear me at the back of the room?"
"She can speak four languages"
She has the skill to communicate in four languages
"She can speak four languages fluently, including Japanese."
"It may rain later"
There is a chance it will rain
"It may rain later, so bring an umbrella just in case."
"That might be true"
It is possible that this is true
"That might be true — I haven't checked the facts yet."
"She may not come"
It is possible she will not attend
"She may not come to the party if she's feeling tired."
"It could be dangerous"
There is a possibility of danger
"It could be dangerous to hike without the proper gear."
"They might have missed the train"
It is possible they did not catch the train
"They might have missed the train — they left very late."
"Can this be right?"
Is it possible this is correct?
"Can this be right? The price seems far too low."
"It could snow tonight"
Snow is a possibility tonight
"It could snow tonight — the temperature is dropping fast."
"You may be right"
It is possible that you are correct
"You may be right about the deadline — let me double-check."
"May I leave early?"
Do I have permission to leave before the usual time?
"May I leave early today? I have a doctor's appointment."
"You can use my laptop"
I give you permission to use my laptop
"You can use my laptop while yours is being repaired."
"Could I ask a question?"
Do I have your permission to ask?
"Could I ask a question about the assignment?"
"You may not enter"
You do not have permission to enter
"You may not enter the restricted area without a badge."
"Can I open the window?"
Do you mind if I open the window?
"Can I open the window? It's very warm in here."
"Students may use dictionaries"
Students are allowed to use dictionaries during the exam
"Students may use dictionaries during the reading test."
"You can't park here"
Parking is not permitted in this location
"You can't park here — it's a fire lane."
"May we sit here?"
Do you allow us to sit in this place?
"May we sit here, or is this table reserved?"
"You must wear a seatbelt"
Wearing a seatbelt is required by law
"You must wear a seatbelt in the front seat at all times."
"She has to finish the report"
She is required to complete the report
"She has to finish the report before the meeting tomorrow."
"You should apologise"
Apologising is the right or expected thing to do
"You should apologise to her — what you said was hurtful."
"Must I attend?"
Am I obligated to be present?
"Must I attend the entire conference, or just the opening session?"
"You ought to call your parents"
It would be right and proper for you to call
"You ought to call your parents — they haven't heard from you in weeks."
"Employees must sign in"
Signing in is a required procedure for employees
"Employees must sign in at reception every morning."
"You don't have to come"
Attending is not required; it is optional
"You don't have to come if you're not feeling well."
"Should I bring anything?"
Is it expected or advisable that I bring something?
"Should I bring anything to the dinner party on Saturday?"
"She must be tired"
I am almost certain she is tired based on evidence
"She must be tired — she's been working since 6 a.m."
"That can't be right"
I am almost certain that is incorrect
"That can't be right — he left an hour ago, so he should be there by now."
"He must have left already"
I am fairly certain he departed before now
"He must have left already — his coat is gone."
"She can't have known"
It is almost impossible that she was aware
"She can't have known about the plan — she looked genuinely surprised."
"They might have taken the wrong road"
It is possible they made a navigation error
"They might have taken the wrong road — they're half an hour late."
"He could be lying"
There is a possibility he is not telling the truth
"He could be lying — his story keeps changing."
"It must have been expensive"
I am quite sure it cost a lot of money
"It must have been expensive — it's a designer watch."
"This could be the answer"
It is possible this is the correct solution
"This could be the answer — let's test it and see."
"You should see a doctor"
My advice is to visit a medical professional
"You should see a doctor about that cough — it's been going on for weeks."
"You ought to save money"
It would be wise and proper to put money aside
"You ought to save money before you quit your job."
"You shouldn't eat so much sugar"
My advice is to reduce your sugar intake
"You shouldn't eat so much sugar if you want more energy."
"You had better hurry"
It is strongly advisable to move faster or there will be consequences
"You had better hurry — the train leaves in five minutes."
"Should I take the job?"
What is your advice — is taking this job a good idea?
"Should I take the job offer even though it means relocating?"
"You could try meditating"
Meditating is a suggestion worth considering
"You could try meditating — it really helps with stress."
"You should back up your files"
My recommendation is to make copies of your files
"You should back up your files before updating the system."
"You had better not be late"
A strong warning that being late will have negative consequences
"You had better not be late — the boss is already annoyed."
よくある質問
英語の助動詞とは何ですか？
助動詞とは、必要性・可能性・許可・能力・アドバイスなどを表す補助動詞です。主な助動詞は can, could, may, might, must, shall, should, will, would, ought to です。これらの後には動詞の原形が続きます（例：「You must go」であって「You must to go」ではありません）。
「must」と「have to」の違いは何ですか？
「must」は話し手個人の義務感や強い勧めを表すことが多いです（「I must finish this today」）。「have to」は外部のルールや要件を示します（「I have to wear a uniform at work」）。否定形では意味が大きく異なります：「mustn't」は禁止を、「don't have to」は不要であることを意味します。
「may」と「might」はどう使い分けますか？
「may」はより高い可能性（約50%）を、「might」はより低い可能性を表します。フォーマルな英語では、「may」が許可を求めたり与えたりする際の優先形でもあります（「May I come in?」）。日常会話では、多くのネイティブスピーカーが可能性の表現として両者を互換的に使います。
助動詞は過去形でどう使いますか？
助動詞は「have＋過去分詞」の形で過去の意味を表します。例：「She must have left」（過去についてのほぼ確実な推測）、「He could have told me」（過去の能力や非難）、「They might have missed the flight」（過去の可能性）。この構造は英語の会話で非常によく使われます。
助動詞はすべて一度に覚える必要がありますか？
いいえ。最も効果的なアプローチは、機能別に学ぶことです。まず能力（can/could）から始め、次に可能性（may/might）、そして義務（must/have to）へと進みましょう。英語のビデオやポッドキャストで自然なコンテキストの中で助動詞を聞くことは、文法表を単独で勉強するよりもはるかに効果的です。
本物の会話の中で助動詞を聞くことが、自然に使えるようになる最速の方法です。
本物の英語動画で練習する →