Some pundits and Senators have suggested that a former President can be impeached and tried as a private citizen. I don't know if they think this applies to all former presidents, including Clinton, Carter, Bush and Obama, or whether it is applicable only to a president, like Trump, who has just recently left office. But either way, they are simply wrong as a matter of the Constitutional text and meaning. The relevant text of the Constitution reads as follows: "The President, Vice President and all civil officers of the United States, shall be removed from Office on Impeachment for, and Conviction of, Treason, Bribery, or other high Crimes and Misdemeanors." (Article II, Section 4)
Another provision of the Constitution says that an impeached president (or other office holder) may be disqualified "to hold and enjoy any office...." So some are arguing that the Constitutional provisions regarding impeachment should be interpreted to apply to any person who may be eligible to run in the future. Such an absurd interpretation of the Constriction would literally allow millions of ordinary citizens over the age of 35 to be impeached and disqualified from future office holding.
This absurd reading of the Constitution shows how far people are willing to go to prevent President Trump from becoming a candidate in 2024. Such an interpretation of the Constitution would render the impeachment provisions utterly meaningless.
The Framers of the Constitution debated impeachment extensively. It is clear that they intended it to apply only to sitting presidents and other office holders and not to private citizens who previously held that office.
So, there can be no real dispute that President Trump could not be impeached and tried once his term ends.
But what if the House of Representatives impeached him while he was still president, but the Senate tried him after his term had concluded. Obviously the Constitution does not explicitly consider or deal with that unanticipated issue. Nor did the Framers consider it. The Framers did, however, regard impeachment and trial as part of one single process, culminating in removal from office. And so, if removal from office is no longer a possibility it would seem that Congress would have no jurisdiction to impeach.