The President can pardon for past crimes, whether or not they have reached the level of indictment or conviction. Or, that’s current thinking.
One of Ford’s first acts to “heal the country” was to pardon Nixon. Nixon had been impeached by the House, which meant that he would fact trial in the Senate. The votes were there to convict him, but all that would have cost him was the disgrace of being kicked out of office. A big disgrace, I’ll grant.
However, in his pardon, Ford said it was possible (or probable, I don’t remember the exact language) that Nixon would be indicted, and he wanted to foreclose that chapter and move forward.
Some have said that this was not the way the pardon power was supposed to work, since Nixon was not under indictment and had not been convicted. But, in the absence of some huge court case, who knows?
Nixon resigned before articles of impeachment could be brought.
Nixon wasn’t impeached.
Only the Democrats Andrew Johnson and Bill Clinton have been impeached.
Only two.