MUST / MUSTN´T

• THEY DON'T TAKE ¨S¨, ÏNG¨or ¨-ED¨ SUFFIX. Example: She must study harder
• THEY ARE FOLLOWED BY BARE INFINITIVE (WITHOUT TO). Example: You must get a good night's sleep
• MUST refers to 1-obligation, necessity or duty. Example: You must stop when the traffic light is red.

          2- strong advice You must behave yourself at school
          3- positive logical assumptions  They must be the boy's parents (I'm sure they are his parent's)    
          4- To give rules  You must warm up before you start running 5- To say that something is veryimportant  You must eat a good breakfast.  
          5- MUSTN'T: prohibition (It is forbidden to do something/ you are not allowed to do something /it is against the rules /law). Example: You mustn't park here
         

They don't have tenses

Present simple: must/mustn't

Simple past: had to / didn't have to

Future simple: will have to