Expressing Cause in Korean — -아서 vs -(으)니까: The Real Difference

<h2>"Because" in Korean: Two Options, One Trap</h2> <p>In English, "because" works in almost every situation. In Korean, there are two main structures for expressing cause — and confusing them is one of the most common learner mistakes.</p> <p>After 12 years in Seoul, I still occasionally pick the wrong one. But with this guide, you'll understand the logic once and for all.</p> <h2>Structure 1: -아/어서 (Natural Cause)</h2> <p>Add <strong>-아/어서</strong> to the verb stem:</p> <table> <tr><th>Verb</th><th>Stem</th><th>+ -아/어서</th></tr> <tr><td>아프다 (apeuda)</td><td>아프-</td><td>아파서 (apaseo)</td></tr> <tr><td>피곤하다 (pigonhada)</td><td>피곤하-</td><td>피곤해서 (pigonhaeseo)</td></tr> <tr><td>비가 오다 (biga oda)</td><td>오-</td><td>비가 와서 (biga waseo)</td></tr> <tr><td>맛있다 (masitda)</td><td>맛있-</td><td>맛있어서 (masissseoseo)</td></tr> <tr><td>바쁘다 (bappeuda)</td><td>바쁘-</td><td>바빠서 (bappaseo)</td></tr> </table> <h3>Examples</h3> <ul> <li>배가 아파서 병원에 갔어요. (baega apaseo byeongwone gasseoyo) — My stomach hurt, so I went to the hospital.</li> <li>비가 와서 우산을 가져왔어요. (biga waseo usaneul gajyeowasseoyo) — It was raining, so I brought an umbrella.</li> <li>너무 피곤해서 일찍 잤어요. (neomu pigonhaeseo iljjik jasseoyo) — I was too tired, so I went to bed early.</li> <li>한국 음식이 맛있어서 자주 먹어요. (hanguk eumsigi masissseoseo jaju meogeoyo) — Korean food is tasty, so I eat it often.</li> </ul> <h2>Structure 2: -(으)니까 (Subjective Cause / With Commands)</h2> <p>Add <strong>-(으)니까</strong> to the verb stem:</p> <table> <tr><th>Verb</th><th>Stem</th><th>+ -(으)니까</th></tr> <tr><td>아프다 (apeuda)</td><td>아프-</td><td>아프니까 (apeunikka)</td></tr> <tr><td>피곤하다 (pigonhada)</td><td>피곤하-</td><td>피곤하니까 (pigonhanikka)</td></tr> <tr><td>비가 오다 (biga oda)</td><td>오-</td><td>비가 오니까 (biga onikka)</td></tr> <tr><td>시간이 없다 (sigani eopda)</td><td>없-</td><td>시간이 없으니까 (sigani eopseunikka)</td></tr> </table> <h3>Examples</h3> <ul> <li>배가 아프니까 병원에 가세요. (baega apeunikka byeongwone gaseyo) — Since your stomach hurts, go to the hospital.</li> <li>비가 오니까 우산을 가져가세요. (biga onikka usaneul gajyeogaseyo) — Since it's raining, take an umbrella.</li> <li>시간이 없으니까 빨리 갑시다. (sigani eopseunikka ppalli gapsida) — We don't have time, so let's hurry.</li> <li>내일 시험이니까 오늘 공부하자. (naeil siheominikka oneul gongbuhaja) — The exam is tomorrow, so let's study today.</li> </ul> <h2>THE Crucial Difference</h2> <p>Here's the golden rule:</p> <table> <tr><th></th><th>-아/어서</th><th>-(으)니까</th></tr> <tr><td>With statements (I went...)</td><td>✅ OK</td><td>✅ OK</td></tr> <tr><td>With commands (Go!)</td><td>❌ FORBIDDEN</td><td>✅ OK</td></tr> <tr><td>With suggestions (Let's go!)</td><td>❌ FORBIDDEN</td><td>✅ OK</td></tr> <tr><td>With requests (Please...)</td><td>❌ FORBIDDEN</td><td>✅ OK</td></tr> </table> <p><strong>In other words:</strong> when the second part of the sentence is a <em>command</em>, <em>suggestion</em>, or <em>request</em>, you MUST use -(으)니까.</p> <h3>Direct Comparison</h3> <ul> <li>✅ 배가 아파서 병원에 갔어요. (My stomach hurt, so I went to the hospital.) — Statement → OK with -아서</li> <li>❌ 배가 아파서 병원에 가세요. — Command → FORBIDDEN with -아서</li> <li>✅ 배가 아프니까 병원에 가세요. (Since your stomach hurts, go to the hospital.) — Command → OK with -니까</li> </ul> <h2>Other Nuances</h2> <h3>-아/어서 NEVER takes past tense in the first clause</h3> <p>With -아/어서, the first part is never conjugated in past tense:</p> <ul> <li>✅ 비가 와서 집에 있었어요. (It was raining, so I stayed home.)</li> <li>❌ 비가 왔어서 집에 있었어요. (INCORRECT)</li> </ul> <h3>-(으)니까 CAN take past tense</h3> <ul> <li>✅ 비가 왔으니까 집에 있었어요. (Since it rained, I stayed home.)</li> <li>✅ 비가 오니까 집에 있었어요. (Since it's raining, I stayed home.)</li> </ul> <h2>Visual Summary</h2> <table> <tr><th>Criteria</th><th>-아/어서</th><th>-(으)니까</th></tr> <tr><td>Tone</td><td>Neutral, factual</td><td>Subjective, justifying</td></tr> <tr><td>With commands</td><td>❌</td><td>✅</td></tr> <tr><td>With suggestions</td><td>❌</td><td>✅</td></tr> <tr><td>Past tense in 1st clause</td><td>❌</td><td>✅</td></tr> <tr><td>Translation</td><td>...so / and therefore</td><td>Since / given that</td></tr> </table> <h2>Exercises</h2> <p>Choose -아/어서 or -(으)니까:</p> <ol> <li>날씨가 좋___ 산책하러 가요. (The weather is nice, let's go for a walk.)</li> <li>피곤___ 일찍 잤어요. (I was tired, so I went to bed early.)</li> <li>시간이 없___ 빨리 하세요. (There's no time, so hurry up.)</li> <li>한국 음식이 맛있___ 자주 먹어요. (Korean food is tasty, so I eat it often.)</li> </ol> <p><strong>Answers:</strong></p> <ol> <li>좋<strong>으니까</strong> → suggestion (가요 = let's go)</li> <li>피곤<strong>해서</strong> → simple statement</li> <li>없<strong>으니까</strong> → command (하세요)</li> <li>맛있<strong>어서</strong> → simple statement</li> </ol> <h2>Want to Master These Nuances?</h2> <p>The difference between -아서 and -(으)니까 is exactly the kind of subtlety that separates a beginner from an intermediate speaker. On <a href="/en/signup">Seonsaengnim</a>, every grammar point is explained with dozens of examples in context. Join us!</p>