The difference is rather subtle. Grammar sites often list it as one of the pitfalls tripping up even the native speakers :-) 1. ~대요: contraction of ~다고 해요. It is a direct quoting. ( A hint for remembering it is the "ㅐ" in "해" which carries over to "대". ) 2. ~데요: a sentence ending forms, indicating a light surprise, exclamation, etc. Therefore, 1. 준석 씨가 아직도 도착 안 했데요. => (I hear/know) Junseok has not arrived yet. => (Guess what / I got to know that / It reached my ear that) Junseok has not arrived yet. 2. 준석 씨가 아직도 도착 안 했대요. = ... 도착 안 했다고 해요. => They say Junseok has not arrived yet. ~데요 has the sense of "guess what..." drawing the listener's attention. The speaker is just bringing up this new topic - he might have heard it or got to know it in a different way. It is more often used in the present tense, as in "날씨가 추운데요", or "차가 좋은데요!", with a light exclamation. Since it is a sentence ending form, ~데요 can only come at the end of a sentence, while ~대요 can be in the middle taking a different form, as in "준석씨가 아직도 도착 안 했다는데 들으셨어요?" (It looks like Junseok has not arrived yet. Did you hear that?").

* Corrections The present tense examples of ~데 I gave, "날씨가 추운데요" and "차가 좋은데요!" are not the same ending forms as used in your example "도착 안 했데요". The former is the -(ㄴ/는)데 which is distinct from the ~데 (or ~더라) we are talking about. Also, the primary function of ~데 and ~더라 is more for relating one's own experience and knowledge, not necessarily a surprise or exclamation. So the two sentences in your question are both mentioning the fact for information. The main difference is that ~대 refers to what one heard or read (quotation) while ~데 may be one's own direct experience or some knowledge that he is confident about. It seems ~대 is much more widely used, and some people often write ~데 by mistake while intending to say ~대 to mean they heard or read it, which makes things even more confusing. So, if all this is a little messing and confusing, I say you at least get a clear grasp of the frequently used ~대 form, and think of ~데 as a similar but more esoteric one.