I've learned that due to Korean syllable structure, every hanja is exactly one hangul block long. Is there any Sino-Korean word (한자어) that deviates from this rule (by colloquial sound change etc.) that could be longer or shorter when written in hangul, or no exception at all?
TL;DR version: While Japanese kanji can be be represented by any number of syllables, Korean hanja always represents a single syllable. Korean words of Chinese origin that have had some sound changes applied are not interchangeable with hanja except in rare cases.
Some words are Chinese in origin but have been nativized through sound changes. Take this common verb for example:
괜찮다 comes from the longer expression 공연하지아니하다 with 공연 coming from the hanja 空然. It probably came to be through common use. The expression became weathered down by the lashing of human tongues over the centuries.
The pronunciation of 공연 merged together to become 괜. While it’s obvious that the ㄱ, ㅗ, and ㄴ were carried over, less obvious are the processes behind the disappearance of the final ㅇ and transformation of ㅕ into ㅐ. I can explain the former quite easily, but the latter is my own guess based on my knowledge of Korean phonology:
- The /ŋ/ sound (ㅇ) in the coda position is susceptible to weakening in many languages. In fact, this is apparent in Chinese borrowings into Japanese. In words where most modern Chinese varieties have /ŋ/, Japanese has a long vowel. 空 is /kɯː/ in Japanese, but /koŋ/ in Korean and Mandarin. Most telling though, is that the null initial and /ŋ/ are both written with the same letter! They were originally written as ㅇ and ㆁ respectively, but still the graphical similarity is no coincidence. Other letters based on the circle shape are ㆆ and ㅎ, with the former being deleted in most cases as in 일 (one) which had been written with the sequence ㆆㅣᇙ.
- The ㅗ can only combine with a limited number of following vowels because of vowel harmony: ㅣ and ㅏ. And because ㅣ can only be at the beginning or end of a diphthong/triphthong, the iotized vowels (ㅑ, ㅕ, ㅛ, and ㅠ) cannot follow ㅗ. I believe, the ㅕ first underwent progressive metaphony to become ㅑ before metathesizing to ㅐ. Of course, 궨 might have also been an equally possible outcome but the ㅏ in the following syllable might have had an influence.
하지아니하 became 찮 through the following mergers:
- 하지 became ㅊ (ㅎ+ㅈ=ㅊ)
- The ㅏ was elided.
- The ㅈ—being a palatal consonant—absorbed the ㅣ—a “palatal” vowel. In modern Korean transliterations of foreign words that end in a palatal sibilant-like sound, Korean employs ㅣ as an epenthetic vowel. “Image” is transliterated as “이미지” since ㅣ produces the least audible sound after ㅈ. Chinese and Japanese borrow such words in a similar manner.
- 아 was preserved as ㅏ.
- 니 became ㄴ.
- 하 became ㅎ.
Another word in Korean that has Chinese origins is 빗 (comb). This was borrowed from Old Chinese 篦 back when it was still pronounced something like /pis/. Today, the sounds have changed in both languages with /pis/ becoming /pit/ in Korean and /pi/ (Pinyin: bì) in Mandarin Chinese. And the hangul for 篦 is 비. 빗 does not have a hanja representation even though its hanja origin is clear.
And now, to really answer the question: words like 괜찮다 are never written with hanja even in mixed-script mode. The fact that they had origins in hanja is usually left as footnotes in a dictionary. So without exception, the one-to-one rule stands.
(Other such examples likely exist. It is even a phenomenon in some Chinese dialects!)
Korean is phonogram:
a unit symbol of a phonetic writing system, standing for a speech sound, syllable, or other sequence of speech sounds without reference to meaning.
While Chinese is ideogram:
a written symbol that represents an idea or object directly rather than a particular word or speech sound, as a Chinese character.
There is no Chinese character that cannot be pronounced using one Hangul block and the Korean pronunciation of Chinese characters is different from pinyin.
This question is not about Chinese, yet in Chinese there are a few "ligature characters" that are used as a shorthand for certain words; for example 圕 for 圖書館 túshūguǎn "library", 瓩 for 千瓦 qiānwǎ "kilowatt", 浬 for 海里 hǎilǐ "nautical mile", 哩 for 英里 yīnglǐ "(English) mile", etc. https://commons.wikimedia.org/wiki/Category:CJK_ligatures and its three subcategories have some other ligature characters that don't seem to be in Unicode.
Also, unless in formal style, the rare numerals 廿 niàn "20", 卅 sà "30", 卌 xì "40", 皕 bì "200" are usually pronounced as if they were written 二十 èrshí, 三十 sānshí, 四十 sìshí, and 二百/两百 èrbǎi/liǎngbǎi.
I don't have a super-large Korean dictionary, but it is conceivable that at least one of these characters is (or was) also used in Korean and read using more than one syllable. Perhaps someone here might want to check.