"The school where I went is expensive."
Translation:제가 가던 학교는 비싸요.
Oh, so you are introducing -던 at the same time as normal past participles without the -더-? Personally I would prefer it to be taught separately at a later point because the feeling it expresses takes quite a bit of getting used to for European language speakers. Or maybe introduce -던 as a special form for 있다 and 형용사 but not yet for 동사 where confusion between -던 and -은/ㄴ arise.
-더- is a suffix whose meaning is quite difficult to explain. It basically adds the feeling that the verb (or adjective) is a recollection of something. Naturally that something is usually in the past (I have heard people argue that it doesn't necessarily have to be the past and that the experience may still be ongoing when -더- is combined with certain endings, but the text in question was talking about some very minor contrasts between certain combinations of endings which for most native speakers have the exact same meaning).
-더- can combine with a number of verb (/adjective) endings but one of the most common combinations is -더- + the past participle ending -은/ㄴ = -던. The difference between -던 and plain -은/ㄴ is next to impossible to translate however: -던 adds the feeling that the speaker is remembering something. For example in the sentence above, -던 expresses that the speaker is talking from memory, something in the direction of “the school I remember going to”, not necessarily about the objective fact that they went there. They could also have said 제가 간 학교, although that sounds somewhat less natural to me in this context since they are talking about their own past experience. Another example:
- 내가 읽은 책 ‘the book(s) I read (objectively speaking)’
- 내가 읽던 책 ‘the book(s) I read (speaking about my memory of reading them)’
The problem is, certain words – namely all adjectives as well as 있다 and 없다 – can only take -던 and never plain -은/ㄴ (or rather, adjectives can take -은/ㄴ but for them that is the present participle, not the past):
- *회사에 있은 컴퓨터 [wrong]
- 회사에 있던 컴퓨터 ‘the computer(s) at the office [lit: at the company]/the computers (that I remember) being at the office’
- 좋은 음식 ‘the food that is good/good food’
- 좋던 음식 ‘the food that was (that I remember being) good’
I’m not quite sure about the reason for this. Maybe historically speaking, adjectives were once verbs which meant “to become [adjective]” (e.g. 좋다 “to become good”), so the plain past participle gave you “the [noun] which has become [adjective]” (e.g. 좋은 음식 “the food which has become good = the good food”). One indication that this may be the case is the way words like 앉다 ‘to sit’, 서다 ‘to stand’, 눕다 ‘to lie down’ are used: They also describe not the state “sitting/standing/lying etc” but the action which you take to enter that state. For example 앉다 means ‘to sit down’, not ‘to be seated’. Consequently, ‘the person sitting next to me’ is 옆자리에서 앉은 사람 (with a past participle, 옆자리 = ‘the seat next to [something]’), literally ‘the person who sat down next to me’. 옆자리에 앉는 사람 (with the present participle) would mean something like ‘the person who is in the process of sitting down next to me’ – which naturally isn’t a very common thing to say. Consequently, if you wanted to talk about the past ‘the person who was sitting next to me’ you would have to resort to -던: 옆자리에서 앉던 사람. But I don’t really know much about Korean historical linguistics so I’m not sure if the same thing was originally true for modern adjectives; it’s just my (not particularly educated) guess about why -은/ㄴ forms a present participle for adjectives rather than a past participle.
In any case, in my opinion -더- is much too advanced a topic for this stage in the course, especially when it comes to its use for normal verbs. It could maybe be given as a special form for adjectives and 있다/없다 because there it is easy to contrast against -은/ㄴ. But introducing it for verbs just leads to confusion. In fact, Duolingo may not be a good medium to teach it in general because the difference in nuance between it and -은/ㄴ cannot really be rendered well in an English translation.