Can a former U.S. president be impeached? Can he be convicted?
Those are two different constitutional questions. And U.S. President Donald Trump, impeached last week while still in office and potentially subject to conviction after departing, has obvious reason to offer a firm "no” to the second question.
Under the Constitution, the House of Representatives is authorized to impeach a president, and then the Senate is authorized to convict him. But that doesn’t answer the questions about a former president.