In short

To join two ideas inside one sentence, stack a connective ending on the first stem: -고 (and), -지만 (but), -거나 (or). All three glue straight onto any stem with no allomorph. To join two separate sentences, start the second one with a matching conjunctive adverb: 그리고, 그래서, 그런데, 하지만 and friends.

The rule

Korean does not have a floating word “and” or “but” that sits between clauses. Instead the connector rides on the end of the first verb or adjective stem, and the sentence keeps flowing to its final verb. When you want the connector to open a fresh sentence instead, you use a conjunctive adverb at the head of that sentence.

Step 1 - The three endings attach with no allomorph

-고, -지만 and -거나 all begin with a plain consonant, so they attach to the bare stem exactly as written - after a vowel, after a 받침, and after a ㄹ-stem alike. There is no 은/는-style vowel/consonant pair to choose, and a ㄹ-stem keeps its ㄹ (unlike before -(으)ㄴ or -ㅂ니다). This makes them among the easiest endings in the language.

• -고 = and - lists things or puts them in sequence: 싸고 맛있어요 (ssago masisseoyo, cheap and tasty).
• -지만 = but - contrasts two ideas: 비싸지만 맛있어요 (bissajiman masisseoyo, expensive but tasty).
• -거나 = or - offers alternative actions: 보거나 쉬어요 (bogeona swieoyo, watch or rest).

Step 2 - -고 has two jobs, and tense behaves differently in each

Listing (A and B, both simply true): each clause may carry its own tense.

• 어제는 비가 왔고 오늘은 눈이 와요. (Eojeneun biga watgo oneureun nuni wayo.) - Yesterday it rained and today it snows. - past 왔고 on the first clause is fine.

Sequence (A, and then B): do not put a tense marker on the first clause. The final verb carries the tense for the whole chain.

• 밥을 먹고 학교에 갔어요. (Babeul meokgo hakgyoe gasseoyo.) - I ate and (then) went to school. - 먹고, not 먹었고, even though the event is past.

Quick test: if you can slip in “and then,” keep the first clause tenseless.

Step 3 - -지만 takes tense freely

Contrast clauses are independent, so -지만 happily follows a past or present stem: present 가지만, past 갔지만 (gatjiman, went but). The tense sits before 지만, on the stem.

• 어제 갔지만 아무도 없었어요. (Eoje gatjiman amudo eopseosseoyo.) - I went yesterday but there was no one.

Step 4 - -거나 is for verbs; nouns use (이)나

-거나 offers a choice between actions: 영화를 보거나 쉬어요 (yeonghwareul bogeona swieoyo, I watch a movie or rest). Its close relative -든지 overlaps with it but adds a whichever, it doesn’t matter shade (마시든지 쉬든지 하세요, drink or rest, whatever). To offer a choice between two nouns, you do not use 거나 - you use (이)나 from lesson 18: 커피나 차 (keopina cha, coffee or tea), where 이나 goes after a 받침 and 나 after a vowel.

Step 5 - Conjunctive adverbs re-package these endings as sentence openers

Most of the “start a new sentence” words are really the little adjective 그렇다/그러하다 (to be so) wearing one of these connective endings and moved to the front. Learn the ending and you get the adverb almost for free:

• 그렇 + -지만 -> 그렇지만 = but, however (하지만 is the very same ending, but on 하- - not on 그렇-)
• 그러 + -(으)면 -> 그러면 (short: 그럼) = then, in that case
• 그렇 + -아/어도 -> 그래도 = even so, still (ㅎ-irregular contraction)
• 그렇 + -아/어서 -> 그래서 = so, therefore

They open a new sentence after a full stop; they never attach to a stem. Use the mapping table below to pair each adverb with the in-sentence ending that means the same thing.

Forms & patterns

The three connective endings - no allomorph, ㄹ-stem keeps its ㄹ:

Because all three start with a consonant, the stem is written just as in the dictionary - the ㄹ-stem does not drop its ㄹ here (contrast 살다 -> 삽니다 / 사세요, where it does).

-고: listing vs sequence and what happens to tense:

Conjunctive adverb <-> matching clause ending:

Examples

Common mistakes

• 밥을 먹었고 학교에 갔어요. (Babeul meogeotgo hakgyoe gasseoyo.) -> 밥을 먹고 학교에 갔어요. (Babeul meokgo hakgyoe gasseoyo.) - For an “and then” sequence, do not mark tense on the first clause; the final verb carries it. (먹었고 is fine only for plain listing, not “then.”)
• 싸다 그리고 맛있어요. (Ssada geurigo masisseoyo.) -> 싸고 맛있어요. (Ssago masisseoyo.) - To join two predicates inside one sentence, stack -고 on the first stem. 그리고 starts a new sentence; it does not attach to a stem, and you never leave the first verb in its dictionary form.
• 커피거나 차를 마셔요. (Keopigeona chareul masyeoyo.) -> 커피나 차를 마셔요. (Keopina chareul masyeoyo.) - -거나 joins verbs/clauses; to say “or” between two nouns, use (이)나 (lesson 18).
• 재미있어요지만 어려워요. (Jaemiisseoyojiman eoryeowoyo.) -> 재미있지만 어려워요. (Jaemiitjiman eoryeowoyo.) - Endings attach to the stem (재미있-), never to a finished 해요체 form. Strip the -어요 before adding -지만.

Tips

• These are your no-fuss endings. -고, -지만, -거나 take no allomorph and trigger no stem change - no 받침 juggling like 은/는 or -(으)면. Just glue them on and keep going to the final verb.
• Most “새 문장” connectors are 그렇다 + an ending. 그렇 + -지만 = 그렇지만, 그러 + -면 = 그러면, 그렇 + -아도 = 그래도, 그렇 + -(으)ㄴ데 = 그런데. (하지만 is the odd one out: it is 하- + -지만.) Learn the in-sentence ending and the sentence-opening adverb comes as a bonus.
• -고 tense trick: if “and then” fits, leave the first clause tenseless (밥을 먹고 갔어요); if it is just “A and B are both true,” tense is free (왔고, 이고).
• 그래서 vs 그런데: 그래서 signals a result (“so”), 그런데 signals a turn or new topic (“but / by the way”). Both are everywhere in speech - reach past 그리고, which only ever means a flat “and.”