She seems to be in a better mood than she was in before.
She seems to be in a better mood than what she was in before.
Are both the above sentences grammatically correct? They have a different grammatical form, which is confusing. What, if anything, is the difference in their meaning?
She seems to be in a better mood than she was in before.
CORRECTShe seems to be in a better mood than what she was in before.
INCORRECT
Than is a conjunction that connects two clauses or coordinates words in the same clause.
In this case, she was in before is the subordinate clause, and more specifically a type of subordinate clause known as a comparative clause, which usually follows the comparative form of an adjective or adverb (in this case better is the comparative form of the adjective good), and begins with than, like, or as (in this case than).
She seems to be in a better mood than she was in before.
CORRECT
In this example, the main clause She seems to be in a better mood is defining the subject as She, the object as better mood, and the verb as seems (where seems to be follows a 'Seem + to-infinitive format and be is the linking verb).
Because those have been defined in the main clause, we do not need to (but can if we want) repeat the process of defining them in the subordinate (comparative) clause, unless we want to change or add meaning.
To illustrate this I'll use two grammatically correct examples, which have slightly different meanings.
She seems to be in a better mood than before.
CORRECT
Meaning: The subject,She, seems to be in a better mood than she seemed to be before.She seems to be in a better mood than she was in before.
CORRECT
Meaning: The subject,She, seems to be in a better mood than she was before.
Notice that by adding she was in, you are changing the meaning because now you're saying that she definitely was in a better mood than she seems to be now, whereas without the she was, you're saying that she seemed to be in a better mood before than she seems to be now.
So, because you defined everything in your main clause, there was no need to do it again in your subordinate clause, but you did, by adding she was, you changed the verb from seem to is, which changed the meaning. Both are grammatically correct, but mean different things.
She seems to be in a better mood than what she was in before.
INCORRECT
In your main clause, She seems to be in a better mood, you are saying that the subject, She, seems to be in the object, which you have defined as mood.
So the reader already knows what she was in, we do not need to define it a second time in the subordinate clause unless we want to change the meaning.
When you say than what she was in, you are saying that she was in something else, something other than a mood, or a good mood, but the reader cannot discern from this sentence what it was that she was in. Was she in a house? In a trance? In a movie? All we know is that she was in a common noun.
By using what in the subordinate clause like that you are explicitly implying she was in something, a thing (common noun), something other than a mood, without actually defining what that thing is, rendering it nonsensical.
This sort of structure is not always incorrect, though, and is used often as a metaphor.
Example:
His car is a hell of a lot better than what he was driving before.
CORRECT
Meaning: The subject,car, is much better than the thing he was driving before.
Notice how in this example, the writer is using the than what structure to refer to an undefined common noun, a thing, that he was driving before. We can assume from the verb driving that whatever the thing was, at least we know it was drivable, but technically speaking, from a grammatical stance, it could be referring to absolutely anything, any common noun, any thing that can be driven (and note that it could be driven according to all definitions of drive, and it could be driven metaphorically). We can't know because the writer is awkwardly leaving it undefined.
In this example, though, we can infer that it is a metaphor used to refer to the subject's previous vehicle as a 'thing', to denote condescension, to denote a sense of inferiority when compared to the better car that he is driving now.
In short: sometimes it can be used correctly, but one has to understand exactly what meaning they are trying to convey before consulting grammar and structuring a sentence correctly.
