Contrast and Background Information: -(으)ㄴ/는데
I have already talked about a few ways to express a contrast in your sentence and I also mentioned a way to add background information to what you are saying with -(으)니까, The grammar ending -(으)ㄴ/는데 is comparable in that way. Like -(으)니까, the grammar ending -(으)ㄴ/는데 can be used to express either a contrast or to add context to what you are saying. The difference between to is a slight nuance you simply need to get used to.
Conjugating with -(으)ㄴ/는데
The grammar ending -(으)ㄴ/는데 is used with both verbs and adjectives. It’s conjugation follows rules you have already seen in topics such as -(으)ㄴ/는/(으)ㄹ 것 같다 and how to use Korean verbs and adjectives attributively. These conjugation rules might seem a bit daunting, but they are manageable. Let us start.
For some grammar points it is important to know what kind of verb you are dealing with.
First you need to know that for the -(으)ㄴ/는데 form, it is important to know whether you are dealing with an adjective or a verb. The conjugation rules for the present tense are a bit different depending this. Luckily you probably know whether a verb is an Korean adjective or an action verb by now.
When you are dealing with either an adjective or the copulative 이다, you will either have either have to attach -ㄴ데 or -은데 to the verb stem for the present tense. You attach -ㄴ데 to all verbs whose verb stem ends with no batchim (final consonant). When the verb does have a batchim, you attach -은데. Quite simple no.
Example: 크다: 크 + -ㄴ데 = 큰데 | 높다: 높 + 은데 = 높은데
The rules for present tense verbs are a bit different. The verb rules also apply to 있다, 없다 and adjectives ending in either of these. For them you simply have to attach -는데 to their verb stem. Korean grammar doesn’t get any easier than that.
Example: 오다: 오 + -는데 = 오는데 | 읽다: 읽 + -는데 = 읽는데
Lastly, it is also possible to use this form with the past tense. When dealing with the past tense, you no longer need to wonder whether something is an adjective or a verb. All verbs follow the same rules in the past tense. The past tense ending of the grammar pattern either -었는데, 았는데 or 했는데 depending the stem. The rules for choosing between either of these three are the rules, I already explained for the past tense.
When you’ve mastered all those tiny rules, you can start conjugating like a true Korean.
The Contrasting -(으)ㄴ/는데
When you use -은/는데 as a way to make contrast in your sentence between clauses, it is a bit like how you use the common -지만 contrast ending. Like that grammar pattern, it is translated as ‘but’ when used to signify contrast. And, as such you use it to indicate a stark contrast between the first and the second clause. You can, however also use it if the second clause has an unexpected result of what you mentioned in the first clause.
Background Info -(으)ㄴ/는데
The second use of the pattern -(으)ㄴ/는데 has a bit more complexity. To really understand it, you will need experience to truly grasp its nuance. You use -(으)ㄴ/는데 also to give some background for whatever comes in the second clause. Therefore you can also use it to simply provide information to introduce the content of the second clause. In English it has a few possible translations because of this such as ‘so’ or ‘therefore’ or even ‘and’.
To help me try to explain what I mean with the paragraph above I will use a simple sentence with -(으)ㄴ/는데:
백화점에 가는데 같이 갈래요? – I’m going to a department store, do you want to come with me?
In this sentence the part 백화점에 가는데 uses the -(으)ㄴ/는데 pattern because it gives information about why I asked the question 같이 갈래요.
However, people use this pattern tremendously often in various situations. Therefore you need to take your time listening so after a time you start understanding how to use this pattern more naturally.