The verb ending -(으)ㄴ/는데요 is a very important and very useful Korean grammar topic beginners ought to learn. This form is often used in daily conversations to express a different thing depending on the situation. It is thus a very flexible verb ending. However, flexibility can end up being confusing.
The first important use for the Korean grammar topic -(으)ㄴ/는데요 is to express your disagreement or opposition to what somebody else just told you. It is a rather polite way to voice your contrary opinion on something without having to use 아니요 (no). You simply inform the other what you think with this verb ending. Because of its nature, when you use it like this, you can never use it in the first sentence of a conversation. You can only use it as a reaction to what others say.
가: 우와 노래가 정말 좋아요.
나: 진짜? 별로인데요.
A: Wow, this song is really good.
B: Really? It’s not that good.
You can use the form -(으)ㄴ/는데요 to express that your surprise when you discover something or when you are feeling something you did not expect. However, unlike its base form, this form expresses the surprise at the end of the sentence. The form itself emphasizes your surprise at what you feel or what you’ve learned.
가: 그림을 끝났어요. 그래서 어때요?
나: 정말 잘했는데요!
A: The painting is done. So what do you think?
B: It’s really well done!
Another use for the form -(으)ㄴ/는데요 is to indicate you are expecting a response from the other person. This particular use is very often used during phone conversations in order to have them run more smoothly. Typically the hidden question you are expecting an answer to is obvious from context. Such as asking for who is calling or the reason for calling.
가:여보세요, 저기 민수 씨 댁이지요?
나: 네, 맞는데요.
A: Hello, this is the home of Minsu, right?
B: Yes, it is…
How To Use
The conjugation of the form -(으)ㄴ/는데요 has the exact same rules as the ones you have to know for its base form -(으)ㄴ/는데. The basics for this conjugation is the ‘batchim/no batchim’ rule you are very familiar with. For the copula form and adjectives in the present tense the form is either -ㄴ데요 (no batchim) or -은데요 (batchim). For verbs, 있다 and 없다, the present tense form is always -는데요.
The past tense forms, however, are the same for both verbs and adjectives. You either add -았는데요 or -었는데요. To decide which form to use, you can just rely on the rules you’ve learned for the Korean past tense conjugation, which is the ‘last vowel’ rule. In both cases, you still need to keep an eye out on the Korean irregular verbs like the ㅎ irregular verbs and the ㅂ irregular verbs.