As a disclaimer, I have read this link, Why does Japanese have two kinds of adjectives? (-i adjectives and -na adjectives) and it could be because it hasn't processed in my brain yet, but I'm not sure I'm closer to some of my answers below. I've never posted here before, so if it is better practice to link me somewhere else if it's already answered my question, that would be great too.
Hi. Just decided to delve deep into the technicalities of Japanese grammar. For reference, I've been watching CureDolly's videos (she says her method is based on Jay Rubin's approach, and she often contrasts with "textbook" information). I'm not sure if the "books" are right, Jay Rubin is wrong, but I'm confused on if this is a black white thing. Is this how Japanese linguists think of Japanese? Or how English linguists think of Japanese? Is this debated? Examples below.
I'm currently in a university Japanese class and just learned about adjectives. Now, I know that there is a distinction between 'Na adjectives" and "I adjectives", and that is predicative forms of na adjectives need a state of being copula to form a complete idea. 花がきれい（だ）。Contrastively, i adjectives have a state of being function built into the "い".
First question, in my previous phrase, an example like 花が赤い forms a complete sentence, and politeness marking is unnecessary. Why and how does the い have a state of being in this scenario?
Continuing, in plain Japanese, I understand that for I adjectives, they "conjugate" like this:
赤い（あかい）to あかく（ない）、あか（かった）、and あかく（なかった）。
Second question, why あかい not use the く in the stem? Does this have something to do with the fact that the state of being is shown with a verb, and non-existence with an adjective? Maybe this has to do with the answer of my first question.
Furthermore, the versions we learn in class (of course being polite), are the same as above with です at the end. What is the distinction between these and the plain Japanese from the last example? Are these just more polite?
Next, when な adjectives are used as a predicate, the polite forms we learn in class can be demonstrated as such:
If I am correct, na adjectives used as a predicate follow the same rules and use the same ending forms as nouns, or at least my textbook says so. Here is an example sentence of two grammatical sentences according to the textbook.
Two questions here. What do these sentences look like in PLAIN japanese? Second, why can I not use ありませんでした in the second sentence?
Thank you very much for your help in my efforts to get better at this interesting language.