Use: We usually use MUST and HAVE TO interchangeably to say that it is necessary to do something. Sometimes, we must use one or the other.
Modals: must, have to
Rule 1: We use
express personal feelings about having to do something.
Example 1: I must call Uncle John this weekend to see how he's doing.
Example 2: Republicans must give up something if there is ever going to be an agreement on debt reduction.
Rule 2: We use
to express facts. Usually, "outside" forces make things
Example 1: Unfortunately, I'm going to miss the party. I have to finish this report before tomorrow.
Example 2: He has to wear safety glasses while welding.
Rule 3: We use
talk about present and future actions.
Example: They must get organized if they want this business to succeed.
Rule 4: MUST NOT and DO NOT HAVE TO have different meanings.
Example 1: He must not take this medication before driving. = For his safety, he cannot take this medication before driving.
Example 2: He doesn't have to take the early train. Let's make it for eleven in the morning. = It is not necessary to leave that early because the meeting will be later in the morning.