Definition
In Korea, word spacing rules are part of the official Hangul spelling rules (Hangul Matchumbeop): a shared convention for separating the words in a sentence by consistent criteria, so that writing is easy to read and hard to misread.
Air Holes in a Sentence: Why Space Out Every Word?
Talk without pausing for breath and nobody can follow you. Letters crammed together tangle up the same way. A famous joke sentence shows it: 아버지가방에들어가신다 (abeoji-gabang-e deureogasinda). Split it as 아버지 가방에 and it means "Father goes into the bag." Split it as 아버지가 방에 and it means "Father goes into the room." One space changes everything.
The big rule of Korean word spacing is simple: put a space between every word in a sentence. A word is roughly the smallest unit that can stand on its own. Words like 하늘 (haneul, sky), 밥 (bap, rice or a meal), 먹다 (meokda, to eat), and 예쁘다 (yeppeuda, pretty) carry their full meaning alone, so each one gets a space on both sides.
Run everything together and your eyes have to hunt for where each word ends, which slows reading way down. Spacing works as a visual comma, helping the reader's brain group the text into chunks and glide through it.
Particles That Cling Like Shadows, and Dependent Nouns That Stand Apart
I said every word gets a space. So why do we write 나는 (naneun, "I" plus a topic marker), 집에서 (jibeseo, "at home"), and 사과를 (sagwareul, "the apple" as an object)? The reason is particles (josa in Korean), small grammar markers that attach after a main word to show its role in the sentence. Particles are officially words, but they can't be used alone. They always cling to the word before them.
So the spelling rules make a special exception: a particle is attached to the word before it. 나 (na, I) + 는 (neun, topic marker); 학교 (hakgyo, school) + 에서 (eseo, at). Even when particles pile up, as in 너에게만은 (neoegemaneun, "even to you, only"), the whole string stays joined.
The opposite trap is the dependent noun, which trips up a lot of people. Words like 것, 수, 줄, 데, 뿐 (geot, su, jul, de, ppun: roughly "thing," "way or ability," "how-to," "place or point," "only") can't stand alone and need a word to modify them. But they are still nouns, which means they are still words, so in principle they are written with a space before them. Leave one gap: 할 수 있다 (hal su itda, "can do"), 먹을 만큼 (meogeul mankeum, "as much as you eat"), 갈 곳 (gal got, "a place to go"). One exception: unit nouns after numbers, like 3년 (3 years) or 10개 (10 items), may also be attached.
A Little More Precisely: Chameleon Words
To be more precise, some Korean words look identical but are particles in one sentence and dependent nouns in another: 뿐, 만큼, 대로, 만 (ppun, mankeum, daero, man: roughly "only," "as much as," "as or according to," and "only" or "after"). The easiest way to tell which is which is to check what comes right before the word.
If a noun or pronoun comes before it (너, 나, 학교, 사과), the word is a particle and attaches: 너뿐이다 (neoppuninda, "it's only you"), 실력만큼 (sillyeokmankeum, "as much as your skill"). If a verb's modifying form comes before it (웃을, 먹을, 생각한), the word is a dependent noun and takes a space: 웃을 뿐이다 (useul ppunida, "all I do is smile"), 노력한 만큼 (noryeokan mankeum, "as much as you tried"). One catch: 만 in 십 년 만에 (sip nyeon mane, "after ten years") marks elapsed time, so it is a dependent noun and gets a space even though a noun comes before it.
Another look-alike is 지. As a dependent noun for time passed, it takes a space: 떠난 지 3년 (tteonan ji 3-nyeon, "three years since leaving"). But the 지 in 갈지 안 갈지 (galji an galji, "whether to go or not") is part of the verb ending -ㄹ지 (-lji), so it attaches to the word before it. It looks complicated, but once you focus on the role a word plays in the sentence, not its shape, you can cut your spacing errors dramatically.
Common misconceptions
Any word that can't stand alone and leans on other words must be attached to the word before it.
Even though they can't stand alone, dependent nouns like 것, 수, 줄, 데, 만큼 (geot, su, jul, de, mankeum) count as separate words and, in principle, take a space before them. Among these leaning words, only particles attach to the word before.
Where you meet it
너뿐이야 (neoppuniya, "it's only you"; particle) is written together, while 바라볼 뿐이다 (barabol ppunida, "I can only watch"; dependent noun) takes a space.
집에 간 지 사흘째 (jibe gan ji saheulje, "it's been three days since going home"; time elapsed) takes a space, while 어디로 갈지 (eodiro galji, "where to go"; verb ending) is written together.
In one sentence
The basic rules of Korean word spacing: put a space between words, attach particles to the word before them, and space dependent nouns away from the word that modifies them.