表示外在規定、情況或客觀需求造成的「必須」。文法結構為 have to + 動詞原形；第三人稱單數用 has to；過去式用 had to；未來常用 will have to。否定與問句使用 do/does/did 助動詞（例：don't/doesn't have to, Do you have to...?）。注意：don't/doesn't have to 表示「不必」，與 mustn't（禁止）不同。
合併形式範例：
I have to finish my homework before 9 p.m.
我必須在晚上九點前完成作業。
其他形式：
has to + V
第三人稱單數主詞（he/she/it 或單數名詞）使用 has to。
had to + V
過去的「必須」。在過去式或過去時間點描述義務或必要性。
will have to + V
表示未來的「必須」。常用於計畫、規定或條件句中。
don't/doesn't have to + V
表示「不必」，即沒有必要（非禁止）。常與 mustn't 對比：mustn't = 不可以／禁止。
have got to + V
口語且多見於英式英語，意義等同 have to（例：I've got to go.）。否定一般不用 haven't got to，而用 don't have to。書寫正式場合建議用 have to。
常見錯誤
✗
You mustn't bring your own water.
✓
You don't have to bring your own water.
mustn't 表示「禁止、不可」，而 don't have to 表示「不必、沒有必要」。當要表達「不需要」時應使用 don't/doesn't have to，而非 mustn't。
✗
He have to do his homework.
✓
He has to do his homework.
第三人稱單數主詞需搭配 has to，而非 have to，為基本主謂一致考點。
✗
I not have to go to cram school.
✓
I don't have to go to cram school.
否定句需使用助動詞 do：主詞 + don't/doesn't + have to + 原形動詞。
✗
Have you to wear a uniform?
✓
Do you have to wear a uniform?
現代英語（特別是美式）問句使用 Do you have to...?；Have you to...? 非標準且少見。
✗
I have to to finish this project.
✓
I have to finish this project.
結構為 have to + 動詞原形，不要重複使用 to。
同義詞與反義詞
同義
must
(必須)
語氣通常比 have to 更強，帶主觀色彩；have to 多指外在規定或客觀需要。過去義務常用 had to 表達，而非 must。
同義
need to
(需要)
語氣較弱，著重「需要」而非規定；否定為 don't need to（= 不必），與 don't have to 意義相近。