Why do Koreans say 웃어른 instead of 윗어른?

ANSWER: Because there is no 아랫어른.

The pure-Korean word 위 means "up" or "top," and the pure-Korean word 아래 means "down" or "bottom," so the word 위아래 can mean either "up and down" or "top and bottom." The words 위 and 아래 need each other to have any real meaning because they need the contrast that each gives to each other. In other words, without an "up" (위), there is no "down" (아래), and without a "top" (위), there is no "bottom." So, when you see a word that uses 위, you can expect that there is a contrasting word that uses 아래.

When Koreans want to create a word that uses the meaning of 윗 without also creating a contrasting word that uses the meaning of 아래, then they use the suffix 웃 instead of 윗 since 웃 does not imply there is a contrasting word that uses the meaning of 아래. In fact, 웃 implies there is no contrasting word that includes the meaning of 아래. So, the word 웃어른 implies there is no such word as 아랫어른. And 웃국 implies there is no such word as 아랫국.

The word 웃어른 literally means "upper (웃) adults (어른)," which translates as "one's elders," and "elders" are adults who are much older than oneself. So, since there is no word that means "lower (아랫) adults (어른)," Koreans use the word 웃어른 instead of 윗어른.

In the above definition, noticed that below the definition for 웃어른, there is a definition for 웃옷. The word 웃옷 is used to refer only to "an outer garment" or "a coat," not to "an upper garment," so the second part of the 웃옷 definition is wrong. For "upper garment," you must say 윗옷 because there is the contrasting word 아래옷, which means "lower garment."

Actually, it seems that Koreans are still trying to figure out when to use 윗 and when to use 웃. For example, my 1998 Donga's Prime Korean-English Dictionary lists both 웃물 and 윗물, but my 2014 Minjung's Essence Korean-English Dictionary lists only 윗물, probably because they have learned that 웃물 would imply there is no 아랫물, but since there is an 아랫물, 윗물 should be used instead of 웃물.

So, just remember this: When there is a 윗, there is also an 아래 or 아랫, but when there is an 웃, there is no 아래 or 아랫, at least there shouldn't be. But some Koreans and even some Korean dictionaries still need time to catch up.

Why is the Korean word for "exchange rate" written as 환율 instead of 환률?

ANSWER: Because the 환 in 환율 (換率) ends in a ㄴ.

In Korea, the Chinese character 率 (률/율) can be written as either 률 or 율, depending on what word precedes it, and it can translate as "rate," "ratio," "proportion," or "percentage," also depending on what word precedes it. If the word that precedes 率 (률/율) ends in a vowel or the consonant ㄴ, then it is written as /율/, as in 비율 (比率), which can translate as "ratio," "rate" or "percentage," or as in 환율 (換率), which means "exchange rate." But if the word that precedes 率 (률/율) ends with any other consonant instead of ㄴ, then it is written as 률, as in 보험률 (保險率), which means "insurance rate"; 성장률 (成長率), which means "growth rate"; or 능률 (能率), which means "efficiency."

The above rules apply not only to the Chinese character 率 (률/율) but also to any Chinese character that can be written as either 률 or 율. So, for example, the Chinese character 律 (률/율), which can translate as "a law," "a regulation," or "commandments," is also written as either 률 or 율 depending on what word precedes it. 도덕률 (道德律), for example, can translate as "moral code," and 자연율 (自然律) can translate as "the natural law." 

The Chinese character 慄 (률/율) means "fear" and can also be written as either 률 or 율, depending on what word precedes it. For example, the Sino-Korean word for "to shudder or to tremble is fear" is 전율 (戰慄), which includes 慄 (률/율), the character for "fear." The reason 전율 is written as "전율" instead of "전률" is that its first syllable 전 ends in a ㄴ, so 전율 also follows the spelling rules mentioned above. By the way, besides meaning "war," the Chinese character 戰 (전) can also mean "to tremble," so the word 전율 (戰慄) can literally translate as "to tremble (戰) in fear (慄)" or "a trembling fear."

Is 찻집 (a tearoom) a pure Korean word?

ANSWER: Yes. Since 2017, the National Institute of Korean Language has claimed that 찻집 is a pure Korean word. That means that they consider 찻집 to be the pure-Korean equivalent of the Sino-Korean word 다방 (茶房), which means "teahouse" or "tearoom." And that suggests that they also consider the correct pronunciation of the Chinese character 茶 to be just /다/, not both /다/ and /차/.

My 2014 edition Minjung's Essence Korean-English Dictionary, however, was published before the National Institute of Korean Language changed its mind on 찻집 and, therefore, lists 찻집 as a combination of the Chinese character for "tea" (茶) and the pure Korean word for "house" (집). That means that Minjung needs to update not only its definition of 찻집 (teahouse) but also its definitions of 찻감 (stuff to make tea out of), 찻물 (tea), 찻숟가락 (teaspoon), 찻잎 (tea leaves), 찻장 (a tea cabinet), and 찻종 (a teacup, a tea bowl) by removing the Chinese character 茶 that appears in the parentheses next to those words. So, the listing for 찻집 should be just "찻집," without parentheses, not "찻집 (茶 --)."

2014 edition of Minjung's Essence Korean-English Dictionary

By the way, there are only six 2-syllable Sino-Korean words that are officially allowed to use 사이시옷 in their spellings. A 사이시옷 is a ㅅ that is sometimes added to a syllable of a compound word to show that the last syllable of that compound word is pronounced differently than its spelling. Normally, the compound words that use 사이시옷s have to be either pure-Korean compound words or a combination of a pure-Korean word and a Chinese character. The six Sino-Korean words below are the only exceptions to that rule:

• 곳간 (庫間) a storeroom, a warehouse
• 셋방 (貰房) a room for rent, a rented room
• 숫자 (數字) a figure, a numeral
• 찻간 (車間) the inside of a train car, a railway carriage
• 툇간 (退間) the wooden floor verandah attached to a traditional Korean house
• 횟수 (回數) the number of times (something is done), frequency

Notice the 찻찬 (茶盞), 찻장 (茶欌), and 찻종 (茶鍾) that are shown in the above photo from my Essence dictionary are not in the above list. If the 찻 in those words was really a Chinese character, then the 사이시옷 in those words would not have been allowed in their spellings since they would have then been considered Sino-Korean words and, therefore, not allowed to use the 사이시옷 in their spellings. But since the 사이시옷 was and is allowed in the spellings of those words, that is evidence that the 찻 in those words is really a pure-Korean word, not a Chinese character.