A practical english grammar - Vyssaja skola 1978

The modal MUST
Sound symbols

a) Necessity, strong obligation

A soldier must always follow orders.

This bill must be paid at once.

Must has no past form. It can be used in subordinate clauses to express necessity or obligation after a past verb, but past necessity in main clauses is expressed by have to. (See Chapter 10.)

The doctor told Bill he must stop eating rich foods.

Bill had to stop eating rich foods.

The negative mustn’t (or must not) does not express the absence of necessity, but rather an emphatic prohibition. The absence of necessity is expressed by not have to.

Pedestrians must not cross the street at this point.