-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. (possibly dated) Synonym of 더라 (-deora).

• In prescriptive grammar, this suffix is the intimate-style equivalent to 더라 (-deora), which is supposed to be plain-style. However, speakers are nowadays increasingly using 더라 (-deora) in the intimate style as well.
• Koreans sometimes confuse this with homophonous 대 (-dae), which is used to mark reported speech:

  그렇데. ― Geureot-de. ― [I personally saw that] it was like that.

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

This confusion may be contributing to its replacement by 더라 (-deora).