Поставьте модальные глаголы в Past Simple или Future Simple Tense.
1. They must be more careful. (Past simple)
2. You may not talk loudly at the meeting. (Past simple)
3. People from poor countries can't go elsewhere looking for higher incomes (Future Simple)
4. The firm can select the output level maximizing its profit (Past Simple)
5. There are a lot of factors in agriculture that must be assumed as risks by the farmers (Future Simple)