A2 · 初中級 チャプター 9

Obligation and Necessity

3 トータルルール
32 例文
5 分
このチャプターのルール クイック練習

Chapter in 30 Seconds

Master the language of rules, duties, and essential choices to navigate daily life with confidence.

  • Distinguish between personal and external necessity.
  • Identify actions that are strictly forbidden.
  • Differentiate between prohibited acts and optional tasks.
Speak with authority and clarity on every obligation.

学べること

Let's explore how to talk about what's truly essential and what's forbidden! By understanding *Must* vs. *Have to* and *Mustn't*, you'll soon be able to clearly express rules, duties, and choices in all your conversations. You've got this!

Learning Objectives

By the end of this chapter, you will be able to:

  1. 1
    By the end you will be able to: Explain classroom or office rules clearly.

チャプターガイド

Overview

Mastering how to express what’s essential and what’s off-limits is a big step in your A2 English grammar journey. This chapter on English obligation and necessity isn't just about learning new words; it's about gaining the power to communicate clearly in everyday situations. Imagine being able to explain a rule, suggest an important action, or politely decline something without confusion.
That’s what we’ll achieve together!
You'll soon understand the subtle yet crucial differences between must and have to, allowing you to describe personal duties versus external rules effortlessly. We'll also tackle how to firmly state prohibitions using mustn't, and how to differentiate between things you absolutely cannot do and things you simply don't need to do with must not vs. need not.
By the end of this guide, you’ll feel confident expressing obligations, necessities, and prohibitions in all your conversations. This isn't just theory; it's practical language that you'll use constantly, whether you're at work, traveling, or just chatting with friends. You're ready to master these essential grammar points and speak with greater precision and confidence!

How This Grammar Works

Let's dive into the core of expressing obligation and necessity. The primary tools you'll use are must and have to, and it's key to understand their unique flavors. We use must when the obligation comes from *inside* you – it's a strong personal feeling, a recommendation you firmly believe in, or something you *personally* decide is necessary.
For example,
I must remember to call my parents today
(it's important *to me*). Remember, never use to directly after must.
On the other hand, have to is for obligations that come from *outside* you – external rules, laws, or requirements from others. Think of it as something you are *forced* or *required* to do. For instance,
Students have to wear a uniform at this school
(it's a school rule, not their personal choice).
It’s very common in everyday conversation.
When you need to say that something is strictly forbidden, prohibited, or a very bad idea, you'll use mustn't. This is a powerful negative command. If a sign says, "You mustn't touch the exhibits," it means touching them is absolutely not allowed.
It’s the strong negative form of must, meaning 'do not do this, it is forbidden.'
Finally, let's look at must not versus need not. While must not is simply another way to say mustn't (e.g.,
You must not interrupt the speaker
– it's forbidden), need not has a completely different meaning. Need not means that something is *not necessary*, and you have a choice.
It's the opposite of having an obligation. For example,
You need not bring a gift to the party, your presence is enough
means bringing a gift is optional, not required. Understanding these distinctions will make your English much clearer.

Common Mistakes

Here are a few common pitfalls to avoid:
  1. 1Using must to: This is a very common error.
✗ You must to study for the exam.
✓ You must study for the exam. (No to after must.)
  1. 1Confusing "mustn't with don't have to": They have opposite meanings. Mustn't is a prohibition; don't have to means no obligation.
✗ We mustn't get up early tomorrow. (This means it's forbidden to get up early, which is usually not what you mean!)
✓ We don't have to get up early tomorrow. (This means it's not necessary to get up early, we have a choice.)
  1. 1Using must for external rules instead of have to: While must can sometimes imply a rule, have to is the standard for external obligations.
✗ All passengers must show their passports at the border. (While technically understandable, have to is more natural for a formal rule.)
✓ All passengers have to show their passports at the border. (This clearly indicates an external requirement.)

Real Conversations

Here are a few examples of these grammar points in action:

1. A: I must remember to send that email before I leave work today. It's really important!

B

B

Yes, and we have to finish this project by Friday, or the client will be unhappy.

2. A: We're going to a museum. Remember, you mustn't run inside.

B

B

Okay, I understand. Do we have to stay together all the time, or can I look at different rooms?

3. A: I made too much food. You need not help me eat it all, but you're welcome to take some home if you like!

B

B

That's kind, but I must not eat too much sugar. I'm trying to be healthy.

Quick FAQ

Q

Can I use must for future obligations?

Yes, you can! For example,

I must go to the dentist next week.
However, have to is often more common and natural for future external obligations, like
I will have to work late tomorrow.

Q

What’s the negative form of have to?

The negative form of have to is don't have to (or doesn't have to for third person singular). It means there is no obligation. For example, "You don't have to come if you don't want to." This is very different from mustn't!

Q

Is must not exactly the same as mustn't?

Yes, they mean exactly the same thing: strict prohibition. Mustn't is simply the common contracted form, especially in spoken English.

Q

When should I use need not instead of don't have to?

They are very similar! Both express a lack of necessity or obligation. Need not can sometimes sound slightly more formal or emphatic than don't have to, but you can often use them interchangeably in A2 contexts.

Cultural Context

Native English speakers use have to far more frequently than must in everyday, informal conversation for obligations. Must often carries a stronger, sometimes more formal or dramatic tone, or expresses a very personal conviction. Mustn't is a strong prohibition and is always understood that way.
For softer prohibitions, speakers might use "shouldn't or can't." Regional differences are minimal in the core meaning, but the frequency of use might vary slightly.

重要な例文 (6)

1

I `must remember` her birthday this year!

今年は彼女の誕生日を覚えておかないと！

Must と Have to: 必要性について話す
2

We `have to submit` our assignment by midnight.

真夜中までに課題を提出しなければなりません。

Must と Have to: 必要性について話す
3

You mustn't post that photo without her permission.

彼女の許可なくその写真を投稿してはいけません。

禁止事項：mustn't（～してはいけない）の使い方
4

We mustn't forget to cancel the subscription before the free trial ends!

無料トライアルが終わる前に、定期購読を解約するのを忘れてはいけません！

禁止事項：mustn't（～してはいけない）の使い方
5

You must not post that photo of me without asking!

私に聞かずにその写真を投稿しちゃダメだよ！

Must Not と Need Not (禁止と選択の違い)
6

You needn't bring any food; I've already ordered pizza.

食べ物は持ってこなくていいよ。もうピザを注文したから。

Must Not と Need Not (禁止と選択の違い)

ヒントとコツ (3)

💡

Internal vs. External

Must は自分の気持ちや信念からくる義務を表すことが多いです。一方、have to は外部のルール、人、状況による義務を示します。例えば、心の中で「もっと勉強しなきゃ」と思うときは、「I must study harder.」
frontend.learn_grammar.from_rule: Must と Have to: 必要性について話す
⚠️

「to」の罠

「mustn't」の後に「to」をつけないでくださいね。例えば、「You mustn't to smoke here.」ではなく、「You mustn't smoke here.」が正しい形です。
frontend.learn_grammar.from_rule: 禁止事項：mustn't（～してはいけない）の使い方
⚠️

「Mustn't」と「Don't Have To」を混同しないで！

「You mustn't come」と言うと、相手を「来ちゃダメ！」と禁止していることになります。「You don't have to come」なら、「来なくてもいいよ」と相手に選択肢を与えています。「You mustn't come.」
frontend.learn_grammar.from_rule: Must Not と Need Not (禁止と選択の違い)

重要な語彙 (5)

obligation something you must do forbidden not allowed essential absolutely necessary duty a moral or legal task optional not required

Real-World Preview

book-open

At the Library

Review Summary

  • Subject + must/have to + verb
  • Subject + mustn't + verb
  • Subject + need not + verb

よくある間違い

Must is a modal verb and is followed by the base form of the verb without 'to'.

Wrong: I must to go now.
正解: I must go now.

We do not use 'do/don't' with 'must'. Use 'mustn't' for prohibition.

Wrong: You don't must go there.
正解: You mustn't go there.

Like must, need not is a modal and does not take 'to' before the main verb.

Wrong: You needn't to pay.
正解: You need not pay.

Next Steps

You have navigated the world of rules perfectly. Keep practicing, and these forms will become second nature to you!

Write a list of 5 rules for your dream country.

Continue to: Present Tenses: Simple, Continuous and Perfect

クイック練習 (9)

間違いを見つけて修正してください。

Find and fix the mistake:

She mustn't to use her laptop in the exam.

✓ Correct! ✗ Not quite. Correct answer: She mustn't use her laptop in the exam.
「mustn't」の後の「to」を削除してください。助動詞の後は直接、動詞の原形が続きます。

frontend.learn_grammar.from_rule: 禁止事項：mustn't（～してはいけない）の使い方

Find and fix the mistake

Find and fix the mistake:

I musted go to the dentist yesterday.

✓ Correct! ✗ Not quite. Correct answer: I had to go to the dentist yesterday.
Must には義務を表す過去形がありません。過去の必要性には had to を使います。

frontend.learn_grammar.from_rule: Must と Have to: 必要性について話す

Which sentence expresses an external obligation?

Choose the correct sentence:

✓ Correct! ✗ Not quite. Correct answer: She has to study for her exam.
試験のための勉強は、通常、コースに合格するための外部からの要件（学校や大学から）です。

frontend.learn_grammar.from_rule: Must と Have to: 必要性について話す

厳格なルールに合う正しい文を選びましょう。

A sign in the hospital says:

✓ Correct! ✗ Not quite. Correct answer: You mustn't smoke here.
病院での喫煙は禁止されているので、禁止を表す「mustn't (must not)」が正しい選択です。

frontend.learn_grammar.from_rule: Must Not と Need Not (禁止と選択の違い)

厳格なルールの場合、どちらの文が正しいですか？

図書館にいる場合、どちらの文が正しいですか？

✓ Correct! ✗ Not quite. Correct answer: You mustn't talk loudly.
「mustn't」の後は、「to」や「-ing」なしで動詞の原形を使います。

frontend.learn_grammar.from_rule: 禁止事項：mustn't（～してはいけない）の使い方

義務がないことを示すように空欄を埋めましょう。

The museum is free today, so you ___ pay for a ticket.

✓ Correct! ✗ Not quite. Correct answer: needn't
美術館が無料なので、支払う義務はありません。「Needn't」は義務がないことを示します。

frontend.learn_grammar.from_rule: Must Not と Need Not (禁止と選択の違い)

正しい形で空欄を埋めてください。

You ______ share your password with strangers.

✓ Correct! ✗ Not quite. Correct answer: mustn't
「mustn't」は禁止や安全ルールに使われます。「don't have to」だと、ここでは危険な「しなくても良い」という意味になってしまいます！

frontend.learn_grammar.from_rule: 禁止事項：mustn't（～してはいけない）の使い方

間違いを見つけて修正しましょう。

Find and fix the mistake:

You must not to forget your password.

✓ Correct! ✗ Not quite. Correct answer: You must not forget your password.
「must」のような助動詞の後には、動詞の原形を「to」なしで使います。

frontend.learn_grammar.from_rule: Must Not と Need Not (禁止と選択の違い)

Choose the correct form

You ___ forget to lock the door when you leave.

✓ Correct! ✗ Not quite. Correct answer: mustn't
ドアを施錠することは必要な安全対策なので、忘れることは禁止です。

frontend.learn_grammar.from_rule: Must と Have to: 必要性について話す

Score: /9

よくある質問 (6)

核となる違いは義務の源です。must は内面的な気持ちや強い信念を表し、have to は外部のルール、法律、状況を表します。例えば、「今日の会議は重要なので、参加しなければならない」という気持ちなら、「I must attend the meeting today.」 と言います。
must は個人の義務、強い推奨、または非常に重要だと感じることを表現するのに最適です。例えば、「今日中に両親に電話しなきゃ」は、「I must call my parents today.」
意味は全く同じですが、「mustn't」は会話やカジュアルな文章で使われる短縮形です。「must not」はよりフォーマルで、強調したい時に使われます。「You must not do that.」
いいえ、それは間違いです。「I mustn't go」が正解です。助動詞の後には「to」をつけないのがルールです。「She mustn't to know」はNGです！
「must not」は「禁止」（してはいけないこと）を意味します。「need not」は「義務がない」（する必要がないこと）を意味します。例えば、「You must not enter」は「入ってはいけません」とドアが閉まっているような状況ですが、「You needn't enter」は「入りたくなければ入らなくてもいいですよ」という意味です。
はい、使えます！現代英語では、「needn't」や「need not」よりも「don't have to」の方が実際によく使われます。どちらも同じ意味ですよ。「You don't have to go.」
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