Both are grammatically correct, but for the sake of flow of the sentence immaturity will do.
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"Immaturity" is the correct term. "Immatureness" is not a widely accepted word in English, so it is better to use "immaturity" to describe the quality or state of being immature.
Immaturity is a noun.
There is no such thing as a "dighthong" in grammar. It is likely a misspelling or a made-up term. If you have a specific grammar question, please provide more information so I can assist you better.
The proper grammar for that sentence is "She was much better than yesterday." This sentence is in the past tense and correctly compares her current state to how she was yesterday.
The prefix"im" indicates the negative of the word maturity and the suffix "ity" indicates a state of being in relation to the adjective "mature", which is the base word of immaturity.
It depends on the situation and wording. Not when writing in a formal tone. Example: He got better at playing the piano. He showed improvement in his piano playing.