4.126 belief: 'must' and 'cannot'

You use 'must' to indicate that you believe something is the case, because of particular facts or circumstances.

• Oh, you must be Sylvia's husband.
• Fashion must account for a small percentage of sales.
• This article must have been written by a woman.

When you are indicating that something is not the case, you use 'cannot'. You do not use 'must not'. (See paragraph 4.130.)

• The two conflicting messages cannot possibly both be true.
• You can't have forgotten me.
• He can't have said that. He just can't.