Pronoun case — nominative (I, she, who), objective (me, her, whom), or possessive (my, her, whose) — becomes tricky in three common situations. In compound constructions, the test is to remove the other person: "between you and me" is correct because "between me" works but "between I" does not. In comparisons with "than" or "as," the implied verb determines case: "She is taller than I [am]." The who/whom distinction follows the same logic: "who" is the subject form (Who called?) and "whom" is the object form (Whom did you call?). Each situation is resolved by the same underlying question — is the pronoun functioning as a subject or an object?
Apply the isolation test for compounds (remove the other noun) and the completion test for comparisons (finish the implied clause). For who/whom, try substituting he/him — if "him" fits, use "whom"; if "he" fits, use "who."
You already know that English pronouns change form depending on their grammatical role: "I" is the subject form, "me" is the object form, "my" is possessive. The rule is clean in simple sentences — "I called her" gets the cases right because there is nothing confusing about a single subject and a single object. The difficulty arises in three specific constructions where speakers routinely misjudge which role a pronoun is playing.
Compound constructions are the most common trap. "He gave it to John and I" sounds formal and educated, but it is wrong. The test is to remove the other person: you would never say "He gave it to I" — you would say "He gave it to me." The compound does not change the grammatical role. The preposition "to" takes an object, and objects take the objective case, whether or not another noun appears alongside. "Between you and me," not "between you and I." "She invited him and me," not "she invited him and I." Remove the companion and the correct case becomes obvious.
Comparisons with "than" or "as" look like complete sentences but are actually elliptical: they have an implied verb that has been omitted. "She is taller than I" is short for "She is taller than I am." The implied verb determines case — "I" is the subject of the implied "am," so the nominative case is correct. But "She likes him more than me" means something different from "She likes him more than I": the first means "more than she likes me" (objective, "me" is the object of the implied "likes"), and the second means "more than I like him" (nominative, "I" is the subject of the implied "like"). Completing the implied clause is not just a grammar test — it disambiguates meaning.
Who and whom follow the same subject-versus-object logic. "Who" is nominative (it is a subject), and "whom" is objective (it is an object). The substitution test makes this concrete: replace the pronoun with "he" or "him" and see which fits. "Who called?" → "He called" — nominative, so "who." "Whom did you call?" → "You called him" — objective, so "whom." In relative clauses the same logic applies, but you must first identify the role the pronoun plays within its own clause, not the surrounding sentence: "She is the one who called" (who = subject of "called"), versus "She is the one whom I called" (whom = object of "I called").
The unifying principle across all three situations is the same: determine whether the pronoun is functioning as a subject or an object in its own grammatical slot, then apply the corresponding case. Sounding formal has nothing to do with it — "between you and I" is wrong whether it sounds impressive or not, because case is determined by grammatical function, not social register.