가다 + ㄴ → 간
The ㄴ has a function equivalent to that or who while specifying a past tense.
Literally, the above reads: person who went to Korea.
Now 가다 is a fairly simple example because its root ends in a vowel. A verb like 먹다 which has a root ending in a consonant takes 은 instead, becoming 먹은.
먹은 판다: panda that ate
Still more confusingly, some consonants behave like vowels, and others like semivowel.
- 팔다 becomes 판. The ㄹ behaves like a silent consonant and is deleted completely. Understanding why this happens to ㄹ (/l/) is another complicated matter though.
- 짓다 becomes 지은. The ㅅ is silenced, and acts as a “hiatus” between the vowels. Historically, this sound would have been weakened to /z/, but then /z/ became weakened—becoming silent.
- 돕다 becomes 도운. The ㅂ weakens into a semivowel /w/, which combines with the weak vowel ㅡ to make ㅜ.
But these irregularities are fairly regular; they are regularly irregular. Nearly all verb roots that end in these consonants will be transformed the same way. A ㄹ will always be replaced with an ㄴ.
Do not confuse the past tense with the present:
- 가다: 간 (past) versus 가는 (present)
- 먹다: 먹은 (past) versus 먹는 (present)
- 팔다: 판 (past) versus 파는 (present)
- 짓다: 지은 (past) versus 짓는 (present)
- 돕다: 도운 (past) versus 돕는 (present)
“The quick and dirty answer is that you use who when you are talking about a person and that when you are talking about an object. Stick with that rule and you'll be safe.” —Mignon Fogarty, Grammar Girl
Never thought you’d find English advice on a Korean-learning course, huh? ;)
A lot of people speak English as a second language and get translations wrong on the English side rather than on the Korean side, then ask why their English translation is wrong. I frequently explain nuances of English on here to help such people. (Why we say "in the sky" or "in heaven" but not "in sky" or "in the heaven" for example--lots of things related to prepositions and articles. But sometimes other topics.)