"She is not fast."
The first one is the Kanji that talks about time as in "early" or the day went by "quickly" but the second one talks about speed like the car is "fast" or I ran "quickly"
早い is a い-adjective, meaning that you need to conjugate it based on what you do with it. If attatching it to a noun, then you can leave it as it is... see example below. 早い車- Fast car
Now if we attatch it to a verb, we need to change the final い to a く. An a example below. 早く走る- run quickly.
Negatives for い-adjectives are based off of ない (negative ある). Because that is a verb (of sorts), we change it first to the adverb form (早い to 早く) and stick on the ない, like in the question. 早くない (not quick)
Past tense of い-adjectives take on かった in place of い. 早い to 早かった. (was quick)
ない is also treated as an い-adjective, so past negative would be... you guessed it! 早くなかった (wasn't quick)
早い is an い-adjective, adjectives in Japanese have their own conjugations just as verbs do. In this case, the negative conjugation of the い-adjective is to drop the い at the end and replacing it with くない, thus it turns into 早くない.
彼女ははやくないです not accepted? Not sure why. It says かのじょははやくないです, I thought 彼女 could be used for both 'she' and 'girlfriend'. As for 'fast', I thought 早く meant early and 速く meant fast. but on other questions every time I have answered 速く for 'fast' it has marked me as being wrong.
Six months later 彼女 is not accepted, which perplexes me as it has been accepted in earlier lessons without a problem.
According to my dictionary, 早い and 速い refer to the same word but 早い is the more used kanji. That might be why 速い is not accepted... but I might be wrong with why Duolingo rejects it.
Other than different kanji, nothing much as they refer to the same word according to my dictionary. 早い is more commonly used over 速い for the adjective はやい.
"Quick before she comes back!" "Oh don't worry dear, she's not fast." is the time I think I will use this sentence.