General rules.
(1) The nominative use is usually marked by the nominative form of the pronoun.
(2) The objective use is usually marked by the objective form of the pronoun.
These simple rules are sometimes violated in spoken and in literary English. Some of the violations are universally condemned; others are generally, if not universally, sanctioned.
Objective for the nominative.
399. The objective is sometimes found instead of the nominative in the following instances:—
(1) By a common vulgarism of ignorance or carelessness, no notice is taken of the proper form to be used as subject; as,—
He and me once went in the dead of winter in a one-hoss shay out to Boonville.—Whitcher, Bedott Papers.
It seems strange to me that them that preach up the doctrine don't admire one who carrys it out.—Josiah Allens Wife.
(2) By faulty analysis of the sentence, the true relation of the words is misunderstood; for example, "Whom think ye that I am?" (In this, whom is the complement after the verb am, and should be the nominative form, who.) "The young Harper, whom they agree was rather nice-looking" (whom is the subject of the verb was).