Sentence Connectors — ~고, ~아/어서, ~지만, ~는데, ~(으)니까

Attach 고 to a verb or adjective stem to list actions or qualities, or to show one action then another in sequence. The tense is marked only on the final clause: 밥을 먹고 잤어요 = 'I ate and (then) slept.' Both clauses can have different subjects. 고 is neutral — it does NOT imply cause.

Add 아서 after ㅏ/ㅗ stems, 어서 after other vowels, and 해서 for 하다 verbs. Two uses: (1) cause — '배가 아파서 못 왔어요' (I couldn't come because my stomach hurt), and (2) sequence where the two actions are tightly linked, often same subject — '집에 가서 잤어요' (I went home and slept there). Key rule: NEVER put past or future tense before 아/어서 — tense goes only on the final clause. You also cannot use it with commands or suggestions.

Attach 지만 directly to a verb or adjective stem to express contrast: 'A but B.' Unlike 아/어서, tense CAN go before 지만: 비쌌지만 샀어요 = 'It was expensive but I bought it.' Works with the copula too: 학생이지만 ('although I'm a student'). It is the cleanest, most direct way to say 'but' inside one sentence.

Use 는데 after verb stems (and 있다/없다), and (으)ㄴ데 after adjectives (예쁜데, 좋은데). It sets up background or context before the main point, and often carries a soft 'but/however' feeling: '오늘은 바쁜데 내일 만날까요?' = 'I'm busy today — how about meeting tomorrow?' Past tense uses 었는데. It is extremely common in conversation for softening and leading into a question or request.

Add 으니까 after a consonant-final stem, 니까 after a vowel-final stem. It gives a reason, but with a more subjective, speaker-justifying nuance than 아/어서. The big advantage: UNLIKE 아/어서, you CAN use 니까 with commands and suggestions — '위험하니까 조심하세요' (It's dangerous, so be careful). Tense can attend before it (했으니까). Use 아/어서 for objective cause-effect; use 니까 when telling someone to do something based on a reason.