"It was I who was wrong" (note, match the tenses, Was / Am is not correct) It sounds ok, like you are correcting someone who thought they were mistaken. other ways you could say this: Actually, it was my mistake The mistake was mine I'm so sorry, i think it was my error and not yours (that caused the problem)

People rarely ever say "It is I who am wrong." As you noted, it is grammatically correct but bookish. Say "I'm the one who's wrong" instead (as you proposed). You definitely hear this more often in everyday speech.

Usually, I would just say, "I'm wrong."

We so rarely admit when we're wrong that there's much debate on how to do it best! I would say one of the following depending upon the circumstances: "I am wrong." "I was wrong." "I made a mistake." "It was my mistake." (In the case where the error was previously discovered and I now admit it was mine.) Note that the connotations of conceding being wrong and making a mistake can differ slightly. Making a mistake might be more forgivable ("Everybody makes mistakes"). However, being wrong, with some matters at least, implies a moral failing or blame instead of mere responsibility.

no