This is something I found scavenging the forum:
Rules of subjective/objective pronoun:
1. If the pronoun is the subject of a sentence/clause, used the subjective form; if its the object, used the objective form.
2. A pronoun that immediately follows a derivate of "to be", such as is, are, was and were, should always be subjective.
3. If a pronoun is the object of a verb or a preposition, then the pronoun needs to be in objective form.
4. Special cases -
If a pronoun immediately follows "but", "except" it should always be objective.
So going by the rules:
Q. No one but him could have told them that the thief was I
A. him could have told them that the thief was I
B. he could have told them that the thief was I
C. he could have told them that the thief was me
D. him could have told them that the thief was me
E. he could have told them the thief was me
I think 'HIM' (Objective form) is necessary as it follows a preposition
& 'I' (Subjective form) is necessary after was.
So it should be A- No one but him could have told them that the thief was I
Test of interchangeability fails here ( As Daagh said)..
I cleared the exam but he failed
I cleared the exam but him failed
According to 4 rule the second option is the correct one
I doubt the validity of these rules.Can any expert validate this question (Souce: Barrons) & the rules.