基本の説明Overview
これから起こることや、予定・意志を表すときは、will か be going to を使います。tomorrow（明日）、next week（来週）、this weekend（今週末）、in the future（将来）のような「未来を表す語句」といっしょによく使われます。To talk about what will happen, or about plans and intentions, use will or be going to. They often appear with future time expressions like tomorrow, next week, this weekend and in the future.
will は「〜するよ」「〜でしょう」のように、その場で決めた意志や、ふつうの予測を表すことが多い言い方です。be going to は、すでに決まっている予定や、今ある根拠から「〜しそうだ」と言うときによく使われます。ただし、どちらを使っても自然な場面も多く、きっぱり分けられるわけではありません。Will often expresses a decision made on the spot (“I'll do it!”) or an ordinary prediction. Be going to is often used for a plan already decided, or for a prediction based on evidence you can see now. In many situations, though, either one sounds natural, so the difference is a tendency rather than a strict rule.
| will | be going to | |
|---|---|---|
| 形Form | will + base verb | am / is / are + going to + base verb |
| よく使う場面Typical use | その場で決めた意志・ふつうの予測On-the-spot decisions, ordinary predictions | 決まっている予定・根拠のある予測Plans already decided, predictions based on evidence |
| 否定文Negative | will not (won't) | am / is / are + not + going to |
| 疑問文Question | Will + subject + base verb ~? | Am / Is / Are + subject + going to + base verb ~? |
| 短縮形Short form | I'll / you'll / he'll / she'll / we'll / they'll | I'm / he's / they're going to |
主語 + will + 動詞の原形 〜. ／ 主語 + am[is, are] + going to + 動詞の原形 〜.Subject + will + base verb ~. / Subject + am[is, are] + going to + base verb ~.will は主語が何でも形は同じ。will のあとの動詞も、going to の to のあとの動詞も、必ず原形にします。Will keeps the same form for every subject. The verb after will, and after the to of going to, is always in the base form.
will は助動詞なので、疑問文は Will を主語の前に出し、否定文は will のあとに not を置きます。be going to の be動詞は主語に合わせて am / is / are と形を変え、疑問文と否定文は be動詞の文と同じように作ります。Will is a modal verb, so a question puts Will before the subject and a negative puts not after will. The be-verb in be going to changes with the subject (am / is / are), and its questions and negatives are made just like those of an ordinary be-verb sentence.
例文で確認Example sentences
① will（意志・予測）1. Will (decisions and predictions)
Oh, that bag looks heavy. I will carry it.
あっ、そのかばん重そうですね。私が持ちますよ。
その場で決めた意志を表す will。Will showing a decision made right at that moment.
I'll call you tonight.
今夜、あなたに電話するよ。
I will は I'll と短縮できる。会話でよく使う。I will can be shortened to I'll, which is common in conversation.
It will be sunny tomorrow.
明日は晴れるでしょう。
天気予報のようなふつうの予測。An ordinary prediction, like a weather forecast.
Mika will be fifteen next month.
ミカは来月15歳になります。
will のあとは原形 be。After will, use the base form be.
She will be a good teacher.
彼女はいい先生になるでしょう。
② be going to（予定・根拠のある予測）2. Be going to (plans and evidence-based predictions)
I am going to visit my grandmother this weekend.
私は今週末、祖母を訪ねる予定です。
すでに決めている予定。主語が I なので am。A plan already decided. The subject is I, so am.
We are going to have a party on Saturday.
私たちは土曜日にパーティーを開く予定です。
Tom is going to study abroad next year.
トムは来年、留学する予定です。
主語が Tom（he）なので is。The subject Tom (he) takes is.
My sister is going to buy a new bike.
姉は新しい自転車を買うつもりです。
Look at those dark clouds! It is going to rain soon.
あの黒い雲を見て。もうすぐ雨が降りそうだよ。
目の前の様子（根拠）から「〜しそうだ」と言うときは be going to がよく使われる。Be going to is common when you predict something from what you can see now.
③ 否定文（〜しないだろう・〜しないつもりだ）3. Negatives
I won't forget your birthday.
あなたの誕生日は忘れません。
will not の短縮形は won't（willn't ではない）。The short form of will not is won't (not “willn't”).
It will not rain tomorrow.
明日は雨が降らないでしょう。
will のあとに not を置く。Put not right after will.
He is not going to join the club.
彼はそのクラブに入るつもりはありません。
be動詞のあとに not を置く。is not は isn't でもよい。Put not after the be-verb. Is not can also be isn't.
We aren't going to go shopping today.
私たちは今日、買い物に行かない予定です。
④ 疑問文（〜するでしょうか・〜する予定ですか）4. Questions
Will it be cold tomorrow?
明日は寒くなるでしょうか。
Yes, it will. / No, it won't. と will を使って答える。Answer with will: Yes, it will. / No, it won't.
Are you going to watch the game tonight?
あなたは今夜、その試合を見る予定ですか。
Yes, I am. / No, I'm not. と be動詞で答える。Answer with the be-verb: Yes, I am. / No, I'm not.
What are you going to do this summer?
あなたはこの夏、何をする予定ですか。
疑問詞 + be動詞 + 主語 + going to + 原形 の順。Word order: question word + be-verb + subject + going to + base verb.
When will Emi come back?
エミはいつ戻ってきますか。
疑問詞 + will + 主語 + 動詞の原形 の順。Word order: question word + will + subject + base verb.
⑤ そのほかの使い方5. Other uses
Will you open the window, please?
窓を開けてくれませんか。
Will you ~? は「〜してくれませんか」とお願いする言い方にもなる。Will you ~? can also be a request: “Would you please…?”
If it is sunny tomorrow, we will play tennis.
もし明日晴れたら、私たちはテニスをします。
if の文の中は、未来のことでも現在形（is）を使う。Inside an if-clause, use the present tense (is) even for the future.
I'm going to be a doctor in the future.
私は将来、医者になるつもりです。
I am は I'm と短縮できる。I am can be shortened to I'm.
確認クイズQuick check
空所に適する語を入れましょう。Fill in each blank with a suitable word.
- Mika ___ be fifteen next month.
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will — あとに原形 be が続くので will。Be follows, so will fits. - We are ___ to visit our uncle next week.
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going — be going to で「〜する予定だ」。Be going to means “plan to.” - My brother ___ going to play baseball tomorrow.
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is — 主語 My brother は単数（he）なので is。My brother is singular (he), so is. - It will ___ cold tomorrow.
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be — will のあとの動詞は原形。is ではなく be。The verb after will is in the base form: be, not is. - ___ you going to cook dinner tonight?
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Are — 主語が you なので Are で始める。The subject is you, so start with Are. - I ___ forget your birthday. （will not の短縮形で）
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won't — will not の短縮形は won't。The short form of will not is won't.
おさえておきたいポイントKey points
- will のあとの動詞は、主語が何でも原形（She will go.）The verb after will is always in the base form, whatever the subject (She will go.)
- be going to の to のあとの動詞も原形。be動詞は主語に合わせて am / is / are にするThe verb after the to of be going to is also in the base form, and the be-verb changes: am / is / are
- will は「その場で決めた意志・ふつうの予測」、be going to は「決まっている予定・根拠のある予測」で使うことが多いWill is often used for on-the-spot decisions and ordinary predictions; be going to for plans already decided and evidence-based predictions
- will の否定は will not（won't）、疑問文は Will を主語の前に出すThe negative of will is will not (won't), and a question puts Will before the subject
- be going to の否定文・疑問文は、be動詞の文と同じ作り方Negatives and questions with be going to are made just like those of be-verb sentences
- tomorrow、next week、this weekend、in the future などの語句が未来のヒントになるWords like tomorrow, next week, this weekend and in the future are hints that the sentence is about the future
よくある間違いCommon mistake："She will goes to the museum." ✗ → "She will go to the museum." ✓ will のあとは、主語が she でも原形です。"She will goes to the museum." ✗ → "She will go to the museum." ✓ After will, use the base form even when the subject is she.
よくある間違いCommon mistake："I going to see him tomorrow." ✗ → "I am going to see him tomorrow." ✓ be going to の be動詞を忘れないようにします。"I going to see him tomorrow." ✗ → "I am going to see him tomorrow." ✓ Don't forget the be-verb in be going to.
よくある間違いCommon mistake："I am will visit Tokyo." ✗ → "I will visit Tokyo." ✓（または I am going to visit Tokyo.）will と be going to を混ぜません。"I am will visit Tokyo." ✗ → "I will visit Tokyo." ✓ (or “I am going to visit Tokyo.”) Don't mix will and be going to.