There are some Korean words which use 'ㅢ' in them such as '띄어쓰기 (word spacing)'.
What is the etymology of '띄어쓰기'? Why can't it be written as '띠어쓰기' and is there any difference between '띄어쓰기' and '띠어쓰기'?
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Originally, it's based on the word 뜨다 which has many meanings, but it seems the meaning that applies is an intransitive verb meaning "to come apart or be apart" (in Korean, the dictionary says "거리가 생기거나 사이가 멀다".
From 뜨다, the causative is formed: 띄우다 ('뜨다'의 사동). "ㅢ" is used rather than "ㅣ" because the original word (뜨다) has an ㅡ in it - the ㅣ우다 is added to this1. One definition of 띄우다 is "사이가 뜨게 하다" - to make something come apart, to separate".
띄우다 is then contracted down to 띄다. The dictionary lists 띄다 as meaning either 뜨이다 or 띄우다.
Finally, 띄다 is combined with 쓰다 (to write) to mean roughly "to separate and write".
1Another example of a word forming the causative this way is 씌우다, from 쓰다. I'm not sure if there are any others with "ㅣ우", but "우" is common; e.g. 깨다 -> 깨우다.