아서 (a-seo, aseo) · 어서 (eo-seo, eoseo) · 니까 (ni-kka, nikka)
아서 and 어서 softly connect cause and effect, while 니까 expresses a more direct and formal cause or reason.
One lane per word — no nested boxes, just the gist.
Used to softly connect the cause or reason of an action or state.
When to use
casual cause connection, soft reason expression, friendly conversation
The nuance
Attach to the verb or adjective stem to indicate cause or reason; tone is soft and conversational.
Similar to 아서 but attaches to stems not ending in 'ㅏ' or 'ㅗ', facilitating natural connection.
When to use
cause connection with non-ㅏ/ㅗ stems, casual reason expression, friendly context
The nuance
Attach to verb or adjective stem; choose 아서 if stem vowel is 'ㅏ' or 'ㅗ', else 어서; indicates soft cause or reason.
Used to state cause or reason directly and clearly, suitable for formal or logical explanations.
When to use
formal reason explanation, logical justification, explanations or information giving
The nuance
Attach to verb or adjective stem to express cause or reason; used in formal or explanatory contexts with a strong tone.
비가 와서 길이 미끄러워요.
bi-ga wa-seo gil-i mi-kkeu-reo-wo-yo., bi ga wa seo gil i mi kkeu reo wo yo.
'오다' stem vowel is 'ㅗ', so 아서 is correctly used for a soft cause connection.
비가 와어서 길이 미끄러워요.
bi-ga wa-eo-seo gil-i mi-kkeu-reo-wo-yo., bi ga wa eo seo gil i mi kkeu reo wo yo.
Since the stem vowel is 'ㅗ', 아서 must be used, not 어서.
춥니까 창문을 닫으세요.
chup-ni-kka chang-mun-eul dat-eu-se-yo., chup ni kka chang mun eul dat eu se yo.
니까 is used for clear, formal cause or reason statements.
춥어서 창문을 닫으세요.
chup-eo-seo chang-mun-eul dat-eu-se-yo., chup eo seo chang mun eul dat eu se yo.
어서 attaches to stems without ㅏ/ㅗ vowel; used for soft cause connection.
배고파서 밥을 먹었어요.
bae-go-pa-seo bap-eul meok-eoss-eo-yo., bae go pa seo bap eul meok eoss eo yo.
아서 is used after stems ending with ㅏ or ㅗ vowel; '배고프다' stem ends with ㅏ sound so 아서 is correct.
배고우니까 밥을 먹었어요.
bae-go-u-ni-kka bap-eul meok-eoss-eo-yo., bae go u ni kka bap eul meok eoss eo yo.
니까 is appropriate for direct cause or reason explanation.
피곤하니까 일찍 자요.
pi-gon-ha-ni-kka il-jjik ja-yo., pi gon ha ni kka il jjik ja yo.
니까 fits formal or clear cause explanations.
피곤하어서 일찍 자요.
pi-gon-ha-eo-seo il-jjik ja-yo., pi gon ha eo seo il jjik ja yo.
After 하다 verbs, 아서 is used, not 하어서.
Which is suitable for soft cause connection?
Which expression is used for formal and clear reasons?
Which attaches after stems with vowels ㅓ or ㅜ?
