1. I looked everywhere for the book but I … find it. A) couldn’t B) mustn’t C) shouldn’t 2. You … drive carefully on a busy road. A) can B) should C) may 3. I don’t feel well today. … I leave a bit earlier? A) may B) must C) should 4. Yesterday I stayed at home because I … help my father. A) can B) must C) had to 5. You … use my dictionary as long as you like. A) must B) may C) have to

I looked everywhere for the book but I couldn't find it. (A) "Couldn't" здесь правильно, потому что это отрицание способности найти книгу в прошлом.

You should drive carefully on a busy road. (B) "Should" выражает совет или рекомендацию.

I don’t feel well today. May I leave a bit earlier? (A) "May" используется для вежливого запроса разрешения.

Yesterday I stayed at home because I had to help my father. (C) "Had to" указывает на необходимость что-то сделать в прошлом.

You may use my dictionary as long as you like. (B) "May" указывает на разрешение.