1. If there is a comparison between two nominative cases, the pronoun of the nominative case is used after ‘than’.
Though he studies hard, he is not quite as smart as her (incorrect)
Though he studies hard, he is not quite as smart as she (correct)
2. A pronoun should be in the nominative case, if a Pronoun is used as the complement of the Verb ‘to be’
If I were her, I’d sign up for that class. (incorrect)
If I were she, I’d sign up for that class. (correct)
3. ” Many a/an” should be followed by a singular noun and singular verb.
Many a politicians have promised to make changes. (incorrect)
Many a politician has promised to make changes. (correct)