MODALS TO EXPRESS OBLIGATION
"Have to", "must", and "should" are used to express obligation.
1. Must expresses a strong obligation:
2. Have to expresses a general obligation based on a law or a rule:
3. Sometimes must and have to are similar:
Sometimes there is a difference. Must is personal. We use must when we express our personal feelings, and say what we think is important or necessary:
Have to is impersonal. It expresses a general obligation based on what another person tells us to do.
4. Have to is used to form the question, and the past and future:
5. Should expresses what is right, or an idea. It expresses advice, or a mild obligation:
Negatives
Mustn't and don't have to expresses very different ideas:
MODALS TO EXPRESS PERMISSION
Can, can't, and be allowed to are used to express permission. Allowed to is passive:
QUESTION: Complete with the most suitable modal verb:
In my school, students ____________ wear uniforms but sometimes we ______________ wear some sport clothes. When students behave in a bad way, their parents ___________ go to school and talk to the Principal about their bad behaviour. My mother doesn't like these people, so she always says that I _____________ get good friends.