-데요 — I recall that..., I saw/felt that...

This expression is used when the speaker is recollecting something that he or she directly learned or felt in the past and is conveying that information to someone else. It is used mainly in spoken language.

Details

★ 말하고자 하는 사실이나 느낌은 이미 과거에 일어난 것이므로 그 시점에서의 과거. 현재 그리고 추측을 말하는 것입니다. (Because the thing or feeling being conveyed by the speaker has already occurred in the past, then the corresponding tense for that particular time—past, present, or supposition—is used.)

1. This expression cannot be used when the speaker is the subject of the sentence.

나는 초등학교 때 친구가 없데요. ✕

However, a first-person subject is allowed if the situation being described is something the speaker did not expect or intend to happen.

그 춤이 중독성이 있어서 (나는) 나도 모르게 따라 하게 되데요.

그 소식을 들으니까 (저는) 갑자기 슬퍼지데요.

2. When the subject is in the second or third person and the predicate is an adjective describing a psychological or emotional state, then the form 형용사+ 아하다/어하다 must be used.

너 아까 우진 씨가 회사 그만둔다고 하니까 기쁘데. ✕

남자 친구가 청혼을 하자 은미 씨는 무척 행복하데요. ✕

3. This expression can be replaced with ‘-더라고요’ and -더군요 with no major change in meaning.

민이가 중국어를 배운 지 얼마 안 됐다고 하던데 꽤 잘하더라고요.

민이가 중국어를 배운 지 얼마 안 됐다고 하던데 꽤 잘하더군요.

While both the forms and pronunciation of -데요 and -대요 are similar and can therefore be confusing, note the following differences between them.