-데

First appears in the late sixteenth century as 데 (Yale: -tey). Traditionally analyzed as a shortening of Middle Korean 더〮ᅌᅵ다〮 (Yale: -té-ngì-tá), from 더〮 (Yale: -té-, imperfective suffix) + ᅌᅵ (Yale: -ngì-, listener-honoring suffix) + 다〮 (Yale: -tá, declarative suffix), with the last being fully deleted.

However, Jang Yun-hui suggests that it may be more appropriate to posit 이 (Yale: -i) as a colloquial Middle Korean verb-final suffix, in which case this is a simple compound of 더〮 (Yale: -té-, imperfective suffix) + 이 (Yale: -i).[1]

데 • (-de)

1. Intimate-style equivalent form of 더라 (-deora).

   걔가 밥을 먹데.

   Gyae-ga bab-eul meok-de.

   [I personally saw that] he was having a meal.

2. The intimate-style interrogative retrospective suffix. See 더 (-deo-) for usage notes.

   걔가 밥을 먹데?

   Gyae-ga bab-eul meok-de?

   [Did you personally see that] he was having a meal?

• Koreans sometimes confuse this with homophonous 대 (-dae), which is used to mark reported speech:

  그렇데. ― Geureo-te. ― [I personally saw that] it was like that.

  그렇대. ― Geureo-tae. ― [According to what people say,] it was like that.