Remember that the first person takes precedence of the second and the second takes precedence of the third. When Cardinal Wolsey said Ego et Rex (I and the King), he showed he was a good grammarian, but a bad courtier.

AM COME—H**E COME

"I am come" points to my being here, while "I have come" intimates that I have just arrived. When the subject is not a person, the verb to be should be used in preference to the verb to have? as, "The box is come" instead of "The box has come."