In the case of what’s taught in the class, it is essentially saying “since __(a time)__”, or talking about how much time has passed since a certain action, or how long a certain action has been going on.

So if you wanted to say “It’s been 8 years since I’ve come to Korea”, the ㄴ지 would mark the “since I’ve come to Korea”. The verb structure ‘to come to Korea’ would be 한국에 오다. Dropping the 다 as we so often do, we are left with ‘한국에 오’ to which we apply the ㄴ지, making it 한국에 온지. Then we add the next part of the sentence, “It’s been 8 years” or “8 years has passed” which is 8년 됐어요. Therefor the whole thing would be 한국에 온지 8년 됐어요.

Another example could be “I’ve been studying Korean for 3 months,” or, “It’s been 3 months since I started studying Korean.” So the duration of time we’re talking about is how long you’ve been studying Korean. The base form ‘to study Korean’ is 한국말 공부하다. Dropping the 다, we’re left with 한국말 공부하. Now adding the ㄴ지 to that, it becomes 한국말 공부한지. Then we want to say how much time has passed, in this case 3 months, we’d say 3개월 됐어요. Putting it all together… 한국말 공부한지 3개월 됐어요.

Does that help at all, Shavonne?