In American speech Degrees of Certainty means how confident you are about how correct a statement is - you are saying.
If you are 100% sure about the topic you are discussing, you must use is. You must be 100% sure the topic is correct. Is, is used for a statement of fact. (Example 1)
If you are 90% sure about the topic you are discussing, the speaker uses must be. (Example 2)
If you are 60% sure about the topic you are discussing, the speaker uses may be, maybe, might be, or could be. These words are discussing suspicious or uncertain thoughts about the topic you are discussing. (Example 3, 4, and 5)
1. Tom is sad.
2. Tom must be sad.
3.Tom may be sad.
4. Maybe tom's sad.
6. Tom might be sad.
7. Tom could be sad.
Must be is also used for when you have complete confidence in a topic you are discussing but you have no idea if its completely true or completely false.
Tom is not here. He is usually in class everyday. Tom must be sick.
*You have no idea if Tom is actually sick or not, but you have complete confidence that he is.