아서/어서/여서 connective ending (because, so)

This pattern connects two separate clauses. The first clause is the reason or the cause. The second clause is the result or the action that follows. Imagine a chain reaction. Clause A happens, which leads directly to Clause B. For example, "It rained, so I stayed home." In Korean, you attach this ending to the verb or adjective in the first part. It acts like a bridge. But wait, there is a second secret use! It also shows a sequence of time. This happens when you go somewhere to do something. Like "I went to the park and met a friend." The two actions are tightly linked. You didn't just go to the park and then randomly meet a friend. You went there, and while there, the meeting happened. It is all about the connection.

1. 1Look at the verb or adjective stem. This is the part before 다.
2. 2Check the very last vowel in that stem.
3. 3If the vowel is ㅏ or ㅗ, add 아서. For example, 가다 (to go) becomes 가서. 오다 (to come) becomes 와서.
4. 4If the vowel is anything else (like ㅓ, ㅜ, ㅣ), add 어서. For example, 먹다 (to eat) becomes 먹어서. 읽다 (to read) becomes 읽어서.
5. 5If the verb ends in 하다, it always changes to 해서. 공부하다 (to study) becomes 공부해서. 요리하다 (to cook) becomes 요리해서.
6. 6For nouns, use 이라서 (if it ends in a consonant) or 라서 (if it ends in a vowel). 학생이라서 (because I am a student).

Use this when you want to give a reason for a feeling. "I am happy because I got a gift." Use it when you are explaining a situation. "I was late because the traffic was bad." It is perfect for daily life scenarios. Imagine you are ordering food. You might say, "This is too spicy, so I can't eat it." Or imagine a job interview. You could say, "I like this field, so I studied hard." It is also the go-to choice for apologies. "I am sorry for being late" is 늦어서 죄송합니다. It sounds sincere and natural. Use it when the reason is a general fact or a common-sense cause. It is like a grammar traffic light. It keeps the conversation moving in the right direction.

This is where many people trip up. Do not use past tense markers like 았/었 before 아서/어서. Even if the whole sentence is in the past, the first part stays in the present form. The final verb at the end of the sentence tells us the timing. Also, never use this pattern with commands or suggestions. You cannot say "It is cold, so wear a coat" using 아서/어서. If you want to tell someone what to do, this grammar is off-limits. It is strictly for stating facts, reasons, and sequences. Think of it as a "no-bossing-people-around" zone. If you try to give an order with it, it sounds very awkward to a native speaker.

One big mistake is saying 갔아서 instead of 가서. You might think, "But I went in the past!" Trust the rule: no past tense in the middle! Another classic error is using it with -세요 (please do). For example, 비가 와서 우산을 가져가세요 is wrong. It sounds like you are trying to mix oil and water. They just don't go together. People also forget the 하다 rule. They might try to say 공부하어서. Remember, 하다 is a rebel. It always transforms into 해서. Finally, don't confuse it with just "and." It is a specific kind of "and" where the first action is necessary for the second one to happen.

You will eventually meet 니까. Both mean "because," but they have different personalities. 아서/어서 is the polite, factual friend. It is used for general reasons and apologies. 니까 is the more assertive, opinionated friend. You use 니까 when you want to give a command, make a suggestion, or state a strong personal opinion. If you say 비가 오니까 우산을 쓰세요, it works perfectly. If you use 아서, it breaks. Also, 아서/어서 shows a tight sequence of events. 고 (and) just lists things like a grocery list. 아서/어서 is more like a movie scene where one action flows into the next.