The origin of the Japanese and Korean languages is still highly
debated, as well as how much they are related. Some linguists estimate that
both languages belong to the Altaic language family, maybe with
influences from Malayo-Polynesian language family. Note that both
languages have nothing in common with Chinese with regard to
grammar.
Korean and Japanese share:
- agglutinative morphology. For example, the verb stem may
be followed by a number of particles and auxiliaries. This is
a trait of Altaic languages.
- a mark of focus. A special grammatical word indicates what
the clause is speaking about (the theme or topic). It may be
different from the subject, as in popular English speaking: "The
car, I've bought it". This is a feature of Altaic
languages.
- word order. Both languages specify the subject
first, followed by the object and the verb. Most Western languages
use the subject-verb-object order. This is another feature of
Altaic languages.
- vocabulary: many words have approximately the same meaning
and pronunciation. Both languages have borrowed a great amount of
vocabulary from Chinese. However, the evolution of pronunciations has
been important enough to make the languages sound quite differently
(incidentally, this is not a criterium for differentiating two languages:
Japanese and Spanish sound closer than English and German to
someone who doesn't understand these languages).
- the writing system: both languages use Chinese
characters. However, Korea nowadays use most of the time a home-made
alphabet, hangul. Japanese itself uses two syllabaries, hiragana and
katakana, in conjunction with the Chinese characters (kanji). Therefore
a Korean text can be immediately distinguished from a Japanese text.
- the absence of the letter r in word initial position: in Korean and Japanese, no native word starts with r, although it's the most frequent Japanese consonant in non-initial position. However, this characteristic is shared by many languages, including Altaic languages and Basque.
Korean and Japanese do not share:
- intelligibility: a Korean will not understand a Japanese, and
a Japanese will not understand a Korean. However, maybe they were able to
understand each other a long time ago, around year 600.
- intonation: Korean intonation tends to rise or remain equal
at the very end of a rhythmic unit, while Japanese intonation tends to fall
or remain equal.
Korean and Japanese share or do not share:
- vowel harmony: both languages used to have it. While vowel harmony is still more or less a feature of the Korean language, the Japanese have abandoned it (more or less).
Thanks tongpoo and izubachi. An interesting source on the letter r in word initial position is http://www.ulb.ac.be/philo/phonolab/r-atics2/abstracts%20pdf/Labrune.pdf.