1. He must go to school every day. 2. You should to drink more water. 3. I can't to find my keys. 4. She may to come to the party. 5. We mustn't doesn't have to do this exercise. It's optional. 6. Children shoulds eat healthy food.

Решение:

• 1. He must go to school every day. (Глагол 'must' требует после себя инфинитив без частицы 'to').
• 2. You should drink more water. (Глагол 'should' требует после себя инфинитив без частицы 'to').
• 3. I can't find my keys. (Глагол 'can'т' требует после себя инфинитив без частицы 'to').
• 4. She may come to the party. (Глагол 'may' требует после себя инфинитив без частицы 'to').
• 5. We don't have to do this exercise. It's optional. (Конструкция 'don't have to' используется для обозначения отсутствия необходимости. 'Mustn't' означает запрет).
• 6. Children should eat healthy food. (Глагол 'should' требует после себя инфинитив без частицы 'to'. 'Shoulds' — неверная форма).

Ответ: 1. must go; 2. should drink; 3. can't find; 4. may come; 5. don't have to; 6. should eat.