He can't have been ill. He must have been tired.#
It can't have been my new hat. It must have been my old one.#
She can't have been Danish. She must have been Swidish.#
He can't have been a dentist. He must have been a doctor.#
She can't have been forty. She must have been fifty.#
It can't have been the 20th. It must be the 21st.#
He can't have been the youngest. He must have been the oldest#
He can't have been reading. He must have been sleeping.&