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While it’s true that the kanji 正 means correct, it’s just a symbol. It’s just glyph that stands for the concept of “correct.” It’s not a word.
The word for correct is ただしい. You can write it with hiragana like that, and that’s totally correct (hehe).
But since 正 is a common kanji that all Japanese people learn right away at school, you usually won’t see it written in just hiragana. You’ll see 正 used to write it, but you can’t just use that symbol alone, because of how Japanese works.
Japanese adjectives and verbs can be conjugated. That is, they can change their form, often based on time (but also for other things).
For instance “is correct” is ただしい, but “was correct” is ただしかった. It’s the same word, but conjugated for time. The end part changed, and so being able to see that end part is pretty important.
Next, if you wrote all of the things that can be expressed with the idea of “correct” as 正 alone, that would get pretty confusing.
There’s also the verb ただす which means “to correct.”
By writing the adjective ただしい as 正しい and the verb ただす as 正す, you can quickly identify which word it is, and you can see that it’s related to “correctness.” You can also see their ends so when they’re conjugated, that element is clear.
There are some words where the whole thing is composed of just the one kanji, but they aren’t いadjectives or verbs, so you never would need to conjugate them.
Studying Japanese grammar will make some of this make more sense, I think.