unfavorable diglossic relationship with Korean, and was largely restricted to informal contexts even between Jeju natives. Within a primarily Jeju-language conversation, speakers might spontaneously switch to Korean to emphasize the rationality or truth value of their statement, while switching to Jeju in a primarily Korean conversation signified that the speaker was making a subjective statement or being less serious. The same study notes that by 1992, even this variety restricted to the informal domain was usually a Korean dialect with a Jeju substratum, rather than the traditional Jeju language: "As for the Jeju language [] in general use nowadays [as of 1992], the situation is that its differences from Standard Korean are greatly diminishing compared to the past. Its greatest differences with Standard Korean [now] lie especially in the suffix paradigm, and in other areas the differences are being minimized. The Jeju people accordingly understand that Jeju and Standard Korean are in a form of dialect continuum, and refer to the native language formerly in use as "thick (or intense) Jeju language" and the Jeju language currently in use as "light Jeju language" or "mixed (with Korean) language." Current status The official language of South Korea is Standard Korean. Nearly all residents of Jeju Island are bilingual in Standard Korean and Jeju, while many younger individuals are even more fluent in English than in Jeju. Standard Korean is most commonly used in the majority of public areas, while Jeju tends to be reserved for use at home and a few local markets. All schools located on Jeju Island are required to teach Standard Korean and only offer Jeju as an elective course. As a result, there are currently no monolingual speakers of Jeju. As of 2018, fluent speakers in Jeju Island were all over seventy years of age, while passive competence was found in some people in their forties and fifties. Younger Islanders speak Korean with Jeju substrate influence found in residual elements of the Jeju verbal paradigm and in select vocabulary such as kinship terms. The language is more vigorous in Osaka, where there may be fluent speakers born as late as the 1960s. Since 2010, UNESCO has classified Jeju as a critically endangered language, defined as one whose "youngest speakers are grandparents and older... [who] speak the language partially and infrequently." A 2008 survey of adult residents' knowledge of ninety Jeju cultural words showed that only twenty-one were