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What's the use of this expression?

Oh, I get it now... We say "She musn't have done that." It's often to make a conclusion based on logic. She said she put yeast in the bread, but the dough isn't rising, so clearly she mustn't have done that. (Because this didn't happen, this must not have (been) done.) Or we use it to forbid something. (Daughter: I'm going to get a tattoo.) (Father: No, you must not do that). It sounds a little old-fashioned to be honest. Normally we just use "should not" or "cannot" to forbid. "Must not have done" is used though when making a conclusion. (You didn't study the test; therefore you must not have done well). It's almost always as a negative conclusion that you are certain of, based on the facts presented by the previous comments. I'm sorry it's so complicated, but it's one of the modal verbs, so it expresses opinion, which you can't make concrete examples of, because it's an abstract concept. If you have trouble understanding this, then I must not have done my job as a teacher well.