Can you impeach a former president ? Can a private citizen be impeached ?
Why does it matter? trump has been impeached (for a second time) before he left office. The trial will happen after he leaves office.
Trials can only be held if there is a material decision to be made. For instance, if a suspect dies, the trial is usually ended. No point in deciding whether someone goes to prison, if they are dead and will not go to prison.
At first, that would sound like the impeachment trial should end. trump would already be removed from office, so why have a trial to decide if he should be removed from office. There is a second thing to decide, whether trump should be banned from office. That is a real material issue.
If Democrats are thinking purely of self interest, they would prefer he be allowed to run again. trump will definitely get the nomination in 2024, and just as certainly lose the general election. That is the perfect combination for Democrats. But Democrats are also thinking of the country, so will probably vote to convict.
If Republicans are thinking purely of self interest, and it were a secret vote, they would vote to convict, for above reasons. trump is tearing the Republican Party apart. The problem is Republicans cannot be seen to be against trump, or they will be destroyed in primaries.
So look to Democrats voting to convict, and Republicans suddenly being called away from town.