-고 — the basic 'and' for verbs and adjectives
-고 is Korean's all-purpose verb/adjective connector. Glue it to a verb stem and the next clause adds either a listed quality ('and also') or a next action ('and then'). 밥을 먹고 학교에 가요 = 'I eat (food) and (then) go to school.' 키가 크고 멋있어요 = 'He's tall and cool.' Pure stem + -고, no vowel-harmony fuss.
How to attach it
Take the verb or adjective stem and add -고 directly. No 받침 rules, no vowel changes. 먹다 → 먹고. 가다 → 가고. 하다 → 하고. 크다 → 크고. 좋다 → 좋고. The connected clause comes second; the final verb carries the tense, register, and ending: 밥을 먹고 학교에 갔어요 = 'I ate and went to school.' (You don't put a tense on 먹고 — only on the last verb.)
Two readings: listing vs. sequence
-고 does two jobs that English splits into 'and' vs. 'and then'. Listing (qualities or simultaneous things): 키가 크고 멋있어요 ('he's tall and cool'), 저는 학생이고 한국 사람이에요 ('I'm a student and Korean'). Sequence (one action, then the next): 밥을 먹고 학교에 가요 ('I eat and then go to school'), 숙제를 하고 잤어요 ('I did my homework and went to bed'). Context picks the reading.
With the copula
이다 before -고 becomes 이고 after a consonant and -고 after a vowel: 학생이고, 친구이고/친구고. So: 저는 학생이고 한국 사람이에요 = 'I am a student and Korean.' Negative copula: 아니고 (학생이 아니고 선생님이에요 — 'not a student but a teacher').
-고 vs. -아서/어서 (cause / tight sequence)
-아서/어서 also connects clauses, but it implies cause ('because, so') or a tight, dependent sequence — the second clause flows out of the first. Compare: 밥을 먹고 학교에 가요 ('I eat, then I go' — two separate things) vs. 배고파서 밥을 먹어요 ('I'm hungry, so I eat' — cause). With places: 학교에 가서 공부해요 ('I go to school and there study' — same trip, dependent) vs. 학교에 가고 도서관에 가요 ('I go to school, and (then/also) I go to the library' — two separate trips). Default to -고 for plain 'and / and then'; reach for -아서/어서 when the clauses are causally or spatially fused.
Negative form
Negate the first verb either with 안 (안 먹고 갔어요 — 'left without eating') or with the longer -지 않고 (먹지 않고 갔어요 — same meaning, slightly more formal). The construction 'V1 -지 않고 V2' is a common way to say 'V2 instead of V1' or 'without V1, V2'.
