When's "must be" used?

Native speaker’s answer
Rebecca
"Must be" is used when you're expressing a thought or opinion that is very likely to be true. Here it's used when the person guesses who the person is, usually based on the information they have from another person or the context. Ex: A: It must be midnight by now. What time is it? B: Five minutes to twelve. You were right! Ex: You must be Charlotte! Jake told me so much about you.