Hanja
Although a phonetic Korean alphabet, now known as Chosŏn'gŭl or Hangul, had been created by Sejong the Great, it did not come into widespread official use until the late 19th and early 20th century. Thus, until that time it was necessary to be fluent in reading and writing Hanja in order to be literate in Korean, as the vast majority of Korean literature and most other Korean documents were written in Literary Chinese, using Hanja as its primary script. Today, a good working knowledge of Chinese characters is still important for anyone who wishes to study older texts (up to about the 1990s), or anyone who wishes to read scholarly texts in the humanities. Learning a certain number of Hanja is very helpful for understanding the etymology of Sino-Korean words, and for enlarging one's Korean vocabulary. Today, Hanja are not used to write native Korean words, which are always rendered in Hangul, and even words of Chinese origin—"Hanja-eo" ()—are written with the Hangul alphabet most of the time, with the corresponding Chinese character often written next to it in order to prevent confusion with other characters or words with the same phonetics.
Korean grammar
Korean is typical of languages with verb-final word order, such as Japanese, in that most affixes are suffixes and clitics are enclitics, modifiers precede the words they modify, and most elements of a phrase or clause are optional.
Sometimes, just using an adverb is insufficient to express the exact meaning the speaker has in mind. The composition of a main verb (or adjective) and a supporting verb (or adjective) can be used in this case, alongside some grammatical features. Suffixes including "-a/eo", "-ge", "-ji", and "-go" are taken by the main verb (or adjective), and the supporting verb (or a.) follows it and is "conjugated".
Korean has general number. That is, a noun on its own is neither singular nor plural. It also has an optional plural marker - "-deul," which is most likely to be used for definite and highly animate nouns (primarily first- and second-person pronouns, to a lesser extent nouns and third-person pronouns referring to humans, etc.) This is similar to several other languages with optional number, such as Japanese.