We love to dig up the past to regale. We often inform others how much time has past since something happened or since we began doing something. Koreans are no different in this love for talking about the past and, of course, they have a Korean time expression just for that: -(으)ㄴ 지.
When we want to indicate how much time has past since a certain situation or action in Korean, we can use the Korean time expression -(으)ㄴ 지. This -(으)ㄴ 지 what you did and in the second clause generally the time that has passed since will be expressed. Hence, most often the second clause will either use the verb 되다 (to become) or 넘다 (To exceed, to be over). The latter, of course, you will often hear in complaints.
가: 언제 부터 한국어를 공부했어요?
나: 한국어를 공부한 지 일 년이 되었어요.
A: Since when did you study Korean?
B: It’s been a year since I’ve been studying Korean.
How To Use
The grammar of -(으)ㄴ 지 is not overly complicated, you just need to keep its rules in mind. This Korean time expression follows the simple, yet well-known Korean grammar principle ‘batchim/no batchim’ with a well-known twist. So when the verb stem ends in batchim (consonant), you use the ending’s form -은 지. However, when it ends in no batchim (vowel), you use -ㄴ 지.
Attention! A verb stem with originally a ㄹ batchim is treated as if it were a vowel. This is important to know for the ㄹ irregular verbs. But, the ㄹ that appears during the ㄷ irregular verb conjugation is treated as any other consonant. Keep these two Korean irregular verb groups in mind when using -(으)ㄴ 지 otherwise you might end up making a few easy to avoid mistakes.
PS: careful observes might have noticed that the -(으)ㄴ part is nothing more than the past tense form of the noun phrase form for verbs.