When do we use "must be"

[englishquestion]

When do we use "must be" is it when we assume something
eg: You must be at work,right.
You must be having dinner

 

[TheParser]

When do we use "must be" is it when we assume something
eg: You must be at work,right.
You must be having dinner

***** NOT A TEACHER *****

Good morning, Englishquestion.

(1) Yes, many times it means that we think something is true:

It's 10 p.m. in New York. My grandmother must be sleeping now.

You have been accepted as a student to the university. You must be very happy.

(2) Sometimes it means something like "should" or "have to":

When you enter a church, you must be quiet.

When you see a yellow traffic light, you must be ready to stop your car.

When you speak with a teacher, you must be respectful.

Have a nice day!

 

[billmcd]

Use "must be" for conclusions as in your examples or "Mary's car is not in the driveway. She must be at work" and for requirements, "You must be quiet in the library".