Il verbo to must significa letteralmente 'dovere' ma si usa in questi casi:

• per esprimere certezza in una supposizione
  esempi:
  This must be the right address!
  It must be him. He told me he would come at three o'clock
  
• per esprimere una proibizione o in generale una cosa che non è raccomandabile fare
  esempi:
  
  you must not park your car here
  you must not swim in the river
  
• per esprimere una forte raccomandazione, un consiglio sentito
  esempi:
  
  You must take some medicine for that cough
  You must see that movie
  You mustn't drink so much
  
• per esprimere un obbligo o una necessità
  esempi:
  
  I must go to the post office tomorrow
  You must have the ticket to enter

Ed ecco un po' di aforismi con must

There must be consistency in direction.
W. Edwards Deming

There must be courage; there must be no awe. There must be criticism, for humor, to my mind, is encapsulated in criticism. There must be a disciplined eye and a wild mind.
Dorothy Parker

We must not promise what we ought not, lest we be called on to perform what we cannot
Abraham Lincoln

We must take the current when it serves, Or lose our ventures
William Shakespeare

Some people never go crazy, What truly horrible lives they must live
Charles Bukowski