When it is necessary or obligatory to do something, you express obligation.
Obligation in the present:
- I have to finish my work before this evening.
- I have got to learn English if I want to live in the USA.
- I must see the doctor soon because I don't feel well.
- It is obligatory that I stop when the traffic light turn red.
- It is necessary that I take a taxi. I'm late.
Obligation in the past:
- I had to work extra hours to pay my bills.
Obligation in the future:
- I'll have to borrow money.
1.The difference between "Have to" and "must" is that "have to"indicates that someone else has imposed conditions on us.
2. The simple past of "must" is "had to".
3. The simple future of "must" is "will have to".
4. Should also expresses obligation. But there is a difference between must / have to and should:
- must / have to = 100% obligation. "When the traffic light turn red, you must stop."
- should = 50% obligation. It's more an advice than an obligation. " You look tired. you should have a rest."
Policeman: Don't you know that you must stop when the traffic lights are red? Driver: Sorry sir. I was so absent-minded that I didn't pay attention to the traffic lights. Policeman: Well sir, you'll have to pay a fine!