In Korean, there are many ways to connect sentences, each with different nuances. With 아/어/여서,(으)니까, there isn’t a specific nuance tied to the situation, but now we will learn about three connective particles which do have specific use cases.
V + 느라/느라고
1. 느라/느라고 shows a negative result with a specific cause. The subjects should be the same, its only used in present tense and you can’t make a ‘let’s sentence using this form.
시험 준비 하느라고 바빠요. – I’m preparing for an exam, so I’m busy. (busy is the negative result).
V + 길래
2. 길래 shows that you are doing an action as a result of observation. When used in a question sentence, you are asking for the reason behind a decision.
비가 오길래 우산을 가져왔어요. – It’s raining so I brought an umbrella. (as a result of observing that it’s raining).
뭐 했길래 이렇게 지쳤어요? – What did you do to be so tired? (asking for details)
V + 더니
3. 더니 is very similar to 길래, except the two actions are either contrasted with one another, or one action is the direct result of the other action.
아침에는 춥더니 지금은 따뜻하네요. – The weather was cold in the morning, and now it’s warm (showing contrast).
V + 았/었/였 + 더니
4. This is the same as 더니 except with past tense forms.
전화를 했더니 친구는 벌써 나가고 없었다. – I called my friend but he already went out.