Incorrect. One can pardon for unindicted — even undiscovered — crimes.
One cannot pardon FUTURE behavior. But Ford pardoned Nixon for unindicted crimes (besided the one he was impeached for, which is a kind of an indictment), and Carter pardoned ALL draft dodgers.
Nixon was never impeached. He resigned before they could vote on it. And then Ford pardoned him.
Sure he can pardon for future crimes, he sings a blank pardon and the dates can be filled in. Read the one of Tom Clancy’s book on this.