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Which one is correct?

My girlfriend is prettier than 'her or she is'.

I think it is 'she is', but is 'her' possible as well? Sounds wrong to me.
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If you use "than" as a preposition, then "her" is the object of that preposition and correctly placed in the objective case.
If you use "than" as a conjunction to introduce the subordinate clause "she is [pretty]," then "she" is the subject of the clause and is correctly place in the nominative case. If you elide the "is," this second case will be understood.

There's no possibility of ambiguity concerning your girlfriend's looks, but there can be in similar constructions:

My girlfriend likes you better than me. (My girlfriend likes you better than she likes me.)
My girlfriend likes you better than I. (My girlfriend likes you better than I like you.)
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deadratQuoteIf you use "than" as a preposition, then "her" is the object of that preposition and correctly placed in the objective case.If you use "than" as a conjunction to introduce the subordinate clause "she is [pretty]," then "she" is the subject of the clause and is correctly place in the nominative case. If you elide the "is," this second case will be understood.
Thanks for you explanation, but it is hard for me to comprehend all this grammar language, as I go with what sounds correct. Do you mean that both 'her' and 'she is' are correct but depending on the intended meaning?

If I want to say that my girlfriend is prettier than the girl over there, should it be "my girlfriend is prettier than she is or her"? I think 'she is' would be the correct choice. Please confirm.

And If I want to say use 'her', should I refer to something that is related to her, as in her sister: "my girlfriend is prettier than her sister". Is that correct?

Is that what you were trying to explain in grammatical terms?

Thanks again!
Anon,

"Her" is not only possible but even probable. In casual/informal speech, most speakers would say "My girlfriend is prettier than her". "You are taller than me". I do not think that anyone would take this to be wrong - I personally wouldn't - so feel free to use it.

That said, the most grammatically sound variant would be:

"My girlfriend is prettier than she is".
No, both sentences are correct

I'm sorry to have confused you with the grammar language, but I encourage you to understand the basics so you don't have to rely on what sounds correct to you. The latter is not only misleading sometimes, but it doesn't help you figure out if you're wrong.

If you want to compare two things or persons (I'll call them A and B) in some quality, you use the comparative form of the adjective associated with that quality and the word "than." Here, the comparative form of "pretty" is "prettier." Here, person A is your girlfriend; person B is another girl. English has two ways to introduce B, and it uses the word "than" in both cases.

The first way is to employ "than" as a preposition governing B, a noun or pronoun. B is said to be the object of the preposition and is in the objective case. English doesn't have different forms for different uses of nouns, so no matter how you used the name "Sally" in a sentence, either as the subject or as an object, "Sally" doesn't change form. Thus

(1) My girlfriend is prettier than Sally. (objective)

(2) Sally is prettier than my girlfriend (nominative)

But it's different for pronouns. If you want to use a feminine pronoun in the objective case, you must use "her," and if you want to use a feminine program in the nominative case, you must use "she." Thus

(3) My girlfriend is prettier than her. (objective)

(4) She is prettier than my girlfriend (nominative)

Sentence (3) is one of the sentences you asked about, and it is correct. (The pronoun must refer to another person. Presumably, you've mentioned this other girl in a previous sentence. Perhaps her name is Sally.) But there's a second way to make the comparison and that's to use "than" to introduce a clause, with it's own subject and verb. Thus

(5) My girlfriend is prettier than she is [pretty].

I put the word "pretty" in brackets it indicate that we leave it out, but that's what we mean. Here B (the thing or person compared to) is the entire clause, and "she" is the subject of the clause ("is," the verb; the understood "pretty," the complement). Subjects of clauses are in the nominative case. But we can also say

(5a) My girlfriend is prettier than she [is pretty].

That is, we leave out both words "is" and "pretty" without changing the meaning. Sentence (5a) is the second of the sentences that you asked about, and it is correct too.

Why would we want to use the the clause form? So we can express ideas like this:

(6) My girlfriend is prettier than she is clever.

Now we're not only comparing people but also different attributes.

Note that sentences (5) and (5a) are not ambiguous. "She" must refer to Sally (or whoever the other girl is) because no one can be prettier than herself. But sentence (6) is ambiguous. It might mean one of two things:

(6a) My girlfriend is prettier than she herself is clever.

(6b) My girlfriend is prettier than Sally is clever.

This is a long way to go to say that both of your sentences are correct, each for its own grammatical reason, but I hope it will help you to analyze sentences on your own. This is not easy stuff, and you are encouraged to return with more questions.
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