-고

From Middle Korean -고〮 (Yale: -kwó), probably from Old Korean 遣, although the phonology is potentially problematic.[1] While Old Korean 遣 corresponds exactly to Middle Korean -고〮 (Yale: -kwó) in terms of grammar, the Middle Chinese pronunciation is 遣 (MC kʰiᴇn^X, kʰiᴇn^H), which does not sound like -고〮 (Yale: -kwó).

In any case, Old Korean 古 (*-kwo) is also, albeit rarely, attested as a connective suffix;[1] either 遣 and 古 are different orthographies of the same suffix (more likely), or the latter subsumed the role of the former. In Middle Korean Idu script, 遣 always denotes -고〮 (Yale: -kwó).

고 • (-go)

1. and then, after...; marks successive action.

   공부하고 놀아야지. ― Gongbu-hago nor-ayaji. ― You should play after studying.

   밥을 먹고 갈게요. ― Bab-eul meokgo galge-yo. ― I'll go after eating.

2. and; connects two clauses together.

   Synonym: (formal) 으며 (-eumyeo)

   운동도 하고 일도 한다. ― Undong-do hago il-do handa. ― [He] exercises, and does work too.

   나는 도착했고 걔가 안 온 거야. ― Na-neun dochakhaetgo gyae-ga an on geo-ya. ― I've arrived; it's him who hasn't come.

3. Used for the main verb to join it to certain auxiliaries.

   하고 있다 ― hago itda ― to be doing

   하고 싶다 ― hago sipda ― to want to do

   하고 나니 ― hago na-ni ― now that it's been done

   하고 들다 ― hago deulda ― to do fiercely

   하고 보니 ― hago bo-ni ― now that I've done it, [I feel...]

   하고 말았다. ― Hago mar-at-da. ― I ended up doing it.

• (and then): Compare 어 (-eo) and 어서 (-eoseo), which have an additional nuance that the first action is impacting or enabling the second.

• 구 (-gu) – dialectal or endearing slang

Contracted from 하고 (hago, “that”, used to quote speech, literally “[one] said, and...”), hence formally identical to the connective suffix above. First attested widely in the Ildong Jang'yu-ga (日東壯遊歌 / 일동장유가), 1763, and other mid-eighteenth-century works.[2][3]

고 • (-go)

1. that; quotative particle marking indirect quotes. Attaches to mood-marking verb-final suffixes of the "plain style".

   자기가 먹었다고 그랬다. ― Jagi-ga meog-eot-da-go geuraetda. ― He said that he ate it.

   내가 어디 가냐고 물었다. ― Nae-ga eodi ga-nya-go mureotda. ― He asked where I was going.

   거기로 가자고 말했다. ― Geogi-ro ga-ja-go malhaetda. ― He said that we should go there.

   왕은 신하에게 자결하라고 명령했다.

   Wang-'eun sinha-ege jagyeolha-ra-go myeongnyeong-haetda.

   The king commanded the minister to take his own life.

2. In the intimate style, a particle attached to verbs to request confirmation of what the speaker has just heard; by extension, can express surprise about what they have just heard.

   내가 먹었다고? ― Nae-ga meog-eot-da-go? ― Did you say I ate it?

   나도 갔냐고? ― Na-do gan-nya-go? ― Did you ask if I went too?

   거기로 가자고요? ― Geogi-ro ga-ja-go-yo? ― Did you say we should go there?

   어디 가라고요? ― Eodi ga-ra-go-yo? ― Where did you say I should go?

3. In the intimate style, a particle attached to verbs to emphasize that the speaker is repeating something they have just said; by extension, can be used to express emphasis or annoyance even if one is not actually repeating something.

   Synonym: 으니까 (-eunikka)

   내가 메달을 땄다고. ― Nae-ga medar-eul ttat-da-go. ― I said, it was me who won the medal.

   너도 좋았냐고. ― Neo-do jo-an-nya-go. ― I asked, did you like it too?

   얼른 가자고. ― Eolleun ga-ja-go. ― I said, let's leave as soon as possible.

   빨리 나가라고요. ― Ppalli naga-ra-go-yo. ― I said, leave quickly.

4. Used with the declarative in certain constructions as 다고 (-dago); see there for more.

• 고 (-go) neutralizes speech level and formality distinctions, and other nuances of the mood-marking suffixes, to the basic suffixes of the "plain style": 다 (-da), 라 (-ra), 냐 (-nya), 으라 (-eura), and 자 (-ja).

Hence declarative sentences are quoted as 다고 (-dago) or 라고 (-rago), interrogative sentences as 냐고 (-nyago), imperative sentences as 으라고 (-eurago), and hortative sentences as 자고 (-jago), regardless of speech level.

가셨습니다. (formal polite style) ― Gasyeotseumnida. ― He went.

가셨다고 합니다. ― Gasyeotda-go hamnida. ― They say that he went.

With the imperative:

이거 먹어. (intimate style) ― Igeo meogeo. ― Eat this.

이거 먹으라고 그랬어. ― Igeo meogeura-go geuraesseo. ― He told me to eat this.

• 고 (-go) can be freely omitted.
• 이라고 (-irago) is used for direct quotes.