Must

MUST
"Must" is most commonly used to express certainty. It can also be used to express necessity or strong recommendation, although native speakers prefer the more flexible form "have to." "Mustnot" can be used to prohibit actions, but this sounds very severe; speakers prefer to use softer modal verbs such as "should not" or "ought not" to dissuade rather than prohibit.
Modal Use | PositiveForms
1. = Present   2. = Past   3. = Future | Negative Forms
1. = Present   2. = Past   3. = Future | You can also use: |
must 
certainty | 1. That must be Jerry. They said he was tall withbright red hair.2. That must have been the right restaurant. There are no other restaurants on this street.3. NO FUTURE FORM | 1. That must not be Jerry. He is supposed to have red hair.2. That must nothave been the right restaurant. I guess there is another one around here somewhere.3. NO FUTURE FORM | have to |
must not 
prohibition |   | You must not swim in that river. It's full ofcrocodiles.You must not forget to take your malaria medication while your are in the tropics.Prohibition usually refer to the near future. | |
must 
strong 
recommendation(Americans 
prefer 
the form "should.") | 1. You must take some time off and get some rest.2. SHIFT TO "SHOULD" 
You should have taken some time off last week to get some rest. 3. SHIFT TO "SHOULD" 
You should take some time offnext week to get some rest.  | 1. You mustn't drink so much. It's not good for your health.2. SHIFT TO "SHOULD" 
You shouldn't have drunk so much. That caused the accident.3. SHIFT TO "SHOULD" 
Youshouldn't drink at the party. You are going to be the designated driver. | should |
must 
necessity(Americans 
prefer 
the form 
"have to.") | 1. You must have a permit to enter the nationalpark.2. SHIFT TO "HAVE TO" 
We had to have a permit to enter the park.3. We must get a permit to enter the park next week.  | 1. SHIFT TO "HAVE TO" 
We don't have to get a permit to enter the national...