45. Obligation: Must vs. Have to

- Internal Obligation: "I must call my mom." (I feel I should). - Written Rules: "Passengers must wear belts."

- External Obligation: "I have to work." (Boss says so).

- Mustn't: PROHIBITION. "You mustn't smoke." (Don't do it!). - Don't Have To: NO OBLIGATION. "You don't have to come." (You can if you want, but it's not needed).