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." "Must not" 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.
Examples:

·       This must be the right address! certainty

·       Students must pass an entrance examination to study at this school. necessity

·       You must take some medicine for that cough. strong recommendation

·       Jenny, you must not play in the street! prohibition

Using "Must" in Present, Past, and Future

Most modal verbs behave quite irregularly in the past and the future. Study the chart below to learn how "must" behaves in different contexts.

Modal Use	Positive Forms 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 with bright 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 not have 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 of crocodiles. 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 off next 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"  You shouldn'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 national park. 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 park. 2. SHIFT TO "HAVE TO"  We didn't have to get a permit to enter the national park. 3. SHIFT TO "HAVE TO"  We won't have to get a permit to enter the national park.	have to

REMEMBER: "Must not" vs. "Do not have to" "Must not" suggests that you are prohibited from doing something. "Do not have to" suggests that someone is not required to do something. Examples: ·       You must not eat that. It is forbidden, it is not allowed. ·       You don't have to eat that. You can if you want to, but it is not necessary.