have to + 動詞は「しなければならない」という意味です。ルールだから、またはほかに選べないからです。
have to + 動詞
I have to be at school at eight.
We have to wear a uniform at work.
Not "I have be at school at eight." — say "I have to be at school at eight.". 動詞の前では to をつけて have to と言います。
やってみよう: Amina: Can I leave these boxes in the hall? Lars: No. The landlord checks the halls every week. We ___ keep them in our apartments. Lars のせりふを選んでください。 (have / have to / don't have to) 答え: have to — 大家さんが毎週廊下をチェックするので、箱を部屋に置くのは自由に選べることではありません。だから Lars は have to + keep と言います。have だけでは to が足りず、don't have to だと自由に選べることになってしまいます。
he, she, it のとき：has to
he、she、it のときは has to と言います。
He has to leave at seven.
The doctor has to see ten people today.
Not "He have to leave at seven." — say "He has to leave at seven.". he, she, it のときは has to と言います。
やってみよう: Amina: Why is the man in 2B so busy? Lars: He ___ move out on Friday. Lars のせりふを選んでください。 (have to / has / has to) 答え: has to — he のときは has to + move と言います。have to は I, you, we, they といっしょに使い、has だけでは to が足りません。
don't have to + 動詞
don't have to + 動詞は「ルールではない」という意味です。してもいいし、しなくてもかまいません。
You don't have to call me. I'm fine.
We don't have to cook tonight.
やってみよう: Amina: The library closes at eight. Is my book late? Lars: No. You ___ take it back today. You have two weeks. Lars のせりふを選んでください。 (don't have to / have to / doesn't have to) 答え: don't have to — Amina にはあと2週間あるので、今日本を返すかどうかは本人が決められます。だから don't have to + take です。have to だと今日が最終日という意味になり、doesn't have to は you ではなく he, she, it といっしょに使います。
he, she, it のとき：doesn't have to
he、she、it のときは doesn't have to と言います。
She doesn't have to wear a uniform.
He doesn't have to work on Friday.
Not "She don't have to go." — say "She doesn't have to go.". he, she, it のときは doesn't have to と言います。
やってみよう: Amina: Does your friend go to a classroom for her English class? Lars: No. She ___ go. The class is online. Lars のせりふを選んでください。 (don't have to / doesn't have to / has to) 答え: doesn't have to — 授業はオンラインなので教室に行く必要はありません。she のときは doesn't have to と言います。don't have to は I, you, we, they といっしょに使い、has to だと教室に行かなければならないことになります。
疑問文：Do you have to? Does he have to?
ルールについてたずねるときは Do you have to + 動詞? と言います。he, she, it のときは Does he have to + 動詞? です。
Do I have to buy a ticket? Yes, you do.
Does she have to study tonight? No, she doesn't.
Not "Does he has to go?" — say "Does he have to go?". does のあとは has to ではなく have to と言います。
やってみよう: Amina: ___ I have to take off my shoes? Lars: Yes, please. The floor is new. Amina の最初のことばを選んでください。 (Does / Am / Do) 答え: Do — I のとき、ルールについての疑問文は Do で始めます：Do I have to take them off? Does は he, she, it といっしょに使い、Am は have to とはいっしょに使いません。
ルールか、自分で選べることか
しなければなりませんか？それなら have to と言います。自分で選べますか？それなら don't have to と言います。
At this school, you have to wear a uniform.
You don't have to wear a tie, but you can.
やってみよう: Amina: Can I go home now? Lars: Sure. You ___ stay. We can do it tomorrow. Lars のせりふを選んでください。 (have to / don't have to / has to) 答え: don't have to — 明日やってもいいので、残るかどうかは Amina が決められます。だから don't have to + stay です。have to だと今夜は残らなければならず、has to は he, she, it といっしょに使います。