レッスン34 · ステージ3: 予定・依頼・比較
義務：must / have to / need to
初級Obligation: must / have to / need to
形と使い方
何かが必要であることを伝える
1.形
何かが必要だと言う方法は3つ：must、have to、need to — どれも + 原形です。must は助動詞で、have to と need to はふつうの動詞と同じように働きます。
| must | have to | need to | |
|---|---|---|---|
| 肯定文 | I / she must go. | I have to go. · She has to go. | I need to go. · She needs to go. |
| 否定文 | mustn't go（= してはいけない） | don't / doesn't have to go（= 必要ない） | don't / doesn't need to go（= 必要ない） |
| 疑問文 | Must I go?（まれ） | Do I have to go? · Does she have to go? | Do I need to go? |
| 過去 | —（had to を使う） | had to go | needed to go |
must には -s も to も do もつきません：She must go. have to / need to はその反対です：She has to go. Does she need to go?
2.どんなときに使うか
1. have to — 規則、仕事、状況のために必要（話し手の意見ではなく事実）
I have to wear a uniform at work.
職場では制服を着なければなりません。
Linh has to take two buses to get to school.
リンは学校へ行くのにバスを2本乗り継がなければなりません。
Do you have to work on Sundays?
日曜日も働かなければいけないの？
2. must — 話し手自身が必要だと感じている：自分へのリマインダーや、強いおすすめ
It's late. I must go.
もう遅い。帰らなきゃ。
I must call my grandma this weekend.
今週末はおばあちゃんに電話しなくちゃ。
You must try the coconut coffee here — it's amazing!
ここのココナッツコーヒーはぜひ飲んでみて。最高だよ！
3. must — 書かれた規則、標識、指示
All visitors must sign in at the front desk.
来訪者は全員、受付で記帳しなければなりません。
Passengers must keep their bags with them at all times.
乗客は手荷物を常に身につけておかなければなりません。
4. need to — 目的のため、または状況のために必要なこと
My phone is almost dead. I need to charge it.
スマホの電池がもうすぐ切れる。充電しなきゃ。
Do we need to book a table, or can we just walk in?
席を予約する必要がある？それとも予約なしで入れる？
5. have got to — have to と同じ意味で、くだけた会話でとてもよく使います
Sorry, I've got to go — my bus is leaving.
ごめん、もう行かなきゃ。バスが出ちゃう。
アメリカ英語では、義務を表す must はかなり強く、かしこまって聞こえます。アメリカ人はふつう have to や have got to（gotta）と言います。迷ったら have to にしておけば、ほぼ間違いありません。
3.ほかの時制では have to を使う
must には形が1つしかなく、現在か未来のことしか言えません。過去、will / might のあと、現在完了形、ほかの動詞のあとでは have to を使います。
| 時制 / 形 | 例 |
|---|---|
| 過去 | I had to wait an hour for the bus. |
| 未来 | Prices go up next month, so we'll have to pay more. |
| Might have to | We might have to sell the car. |
| 現在完了形 | I've had to wear glasses since I was ten. |
| ほかの動詞のあと | I hate having to get up in the dark. |
4.mustn't と don't have to はまったく違う
| 意味 | 例 | |
|---|---|---|
| mustn't | してはいけない — 許されていない | You mustn't smoke in here. It's a hospital. |
| don't have to / don't need to | 必要ない — したければしてもいい | You don't have to bring food. There's plenty. |
You mustn't touch the paintings.
絵に触ってはいけません。
You don't have to take your shoes off. Come in!
靴は脱がなくていいですよ。どうぞ入って！
mustn't = 禁止、don't have to = してもしなくてもいい。混同しないようにしましょう。レッスン「不必要：don't have to / needn't」も見てください。
5.よくある間違い
You must to turn off your phone in class.You must turn off your phone in class.
must のあとに to はつけません。
She have to finish her homework.She has to finish her homework.
he/she/it → has to。
Last week I must work every evening.Last week I had to work every evening.
過去には had to を使います。
Do you must wear a helmet?Do you have to wear a helmet?
must に do は使いません。Do you have to…? とたずねます。
We will must show our passports.We will have to show our passports.
未来：will have to。
She needs wear a uniform.She needs to wear a uniform.
ふつうの動詞としての need → need to + 動詞。
例文
- All students must wear a uniform at this school.
- My brother has to work on Saturdays.
- Did you have to pay for the tickets?
- Yesterday I had to stay late at the office to finish a report.
