It’s one thing not to be overly strict in matters of grammatical correctness (grammar is made for us, after all, not we for grammar), but when what we mean to say depends on how we say it, then our grammar must be exact. Take these two sentences, for example: I drove him to school more often than her and I drove him to school more often than she. Which do you think is correct? The answer depends on one simple master rule of English grammar: two things being compared must be in the same case. But what does that mean?
Grammatical case refers to the way a noun or pronoun changes its spelling or position in a sentence in order to change its grammatical function. Both of the sentences in our example begin with the pronoun I, which is, in fact, only one of three forms for that personal pronoun: I, my, me. These three forms denote the three cases in English, and each tells a reader or listener how to use the word grammatically in a statement. The pronoun I represents the nominative case, which signifies the subject of a clause; the pronoun my represents the possessive case, which signifies the possessor of something; and the pronoun me represents the objective case, which signifies the object of a verb. We say, then, I drove and not me drove because the pronoun is meant to mark the subject of the verb drove. Subjects are in the nominative case, and so I drove is correct and our ear concurs.
If we continue our analysis by adding the third word, the pronoun him, all remains fairly straightforward. Him is the objective case of he, his, him, and the objective case is correct here because the verb drove needs a direct object, the person who got driven, so to speak. The only difference between the two sentences, in fact, is the final word, and that difference is a change of case: her and she are case forms of the pronoun she, her, her. Note that the possessive and objective case forms of this pronoun are identical (her), but we can rule out the possessive case as a possible answer in our example because there is no other thing mentioned that she could be the possessor of: her friends, for example. We are left, then, with the observation that the only difference between the two sentences is that the first ends with the objective pronoun (her) to signify an object, and the second ends with the nominative pronoun (she) to signify a subject.
And now we come again to our master rule: two things being compared must be in the same case. The phrase more often than in our examples denotes a comparison, and so all we have to do is match the identical cases in order to find the comparison being made in each sentence. Since the first sentence ends with the objective pronoun her, that sentence intends to compare how often the subject I drove her compared to how often I drove him, the other objective pronoun in the sentence. Likewise, since the second sentence ends with the nominative pronoun she, that sentence means to say how often she drove him when compared to how often I drove him. The last word of each sentence changes because the comparison being made in each sentence is different.
So which is correct? Both are, depending upon just what the writer wants to say. It’s always a logical error to compare apples and oranges, things of different form and function. In writing, we can keep things straight by understanding the grammatical phenomenon of case, which is more than simply a matter of etiquette. When form changes meaning, more is at stake than merely being proper.