Why does Japanese have two kinds of adjectives? (-i adjectives and -na adjectives)?

Question

Japanese has two kinds of adjectives known by several terms but the ones I know are `i-adjectives` and `na-adjectives` - why? I recall that Japanese adjectives are much more like verbs than in English and most European languages (where they are more closely related to nouns), but does this apply to both types of Japanese adjectives or only one type? I seem to remember that the two kinds are totally different to one another - is this true and if so how did each originate? For instance do we know if Okinawan/Rkyukyuan languages also have two types of adjectives? What about the Korean language, which is the language next most often claimed to be related to Japanese? (though far from universally accepted).

Answer 1

Favorite question 2019.

MODERN Japanese has two kinds of adjectives: `i-adjectives` and `na-adjectives.` Let's look at some linguistic history.

Japanese, like English and any other language, has constantly been evolving. In the modern era, Japanese had 4 types of adjective. -KI, -SHIKI, -TARI and -NARI. Each of these four adjectives had embedded within them the sense of "to be." -KI and -SHIKI adjectives are mostly comprised of native Japanese words, which is also true of their decedents, the -I adjectives. They still contain the sense of "to be" within themselves and DESU is usually added to make the utterance more polite.

both -TARI and -NARI (and a few more words) were also stand alone copulas which were later replaced by DESU, although I think you can make the inference that -NARI itself became NARU, "to become." Also as the spoken language became more simplified and changed through the ages, the written word became updates and simplified due to the "Genbun Itchi" movement. One of the conventions was to use the modern copula (DA/DESU) and eliminate -TARI and -NARI, or at least update their roles in the language.

Unfortunately, I cannot speak to neither the Ryukyu dialect nor Korean, but I hope this is interesting to you. I suggest researching Classical Japanese or taking a course if you're able to. You'll find that it helps you make sense of modern Japanese a bit more.