Korean grammar, 니까 vs 어서 (아/어서 vs (으)니까)

There are lots of expressions regarding the reasons in Korean grammar.

니까(1) and 어서(2) are representative Korean grammar expressing reasons,

but many students have difficulties to figure out the difference between them.

1)The biggest difference is, that 아/어서 cannot be used with imperative(ordering) or proposing clause.

비가 오니까 택시를 타고 와(o)(Take a taxi as it's raining.)
늦었으니까 우리 뛰어가자(o)(Let's run as it's late)
비가 와서 택시를 타고 와(x)
늦어서 우리 뛰어가자(x)


2)The other differences are regarding nuances when you say a sentence using 니까 or 어서.

어서 express common sense or general(objective) reasons of an action or state,
while 니까 talks about personal opinion or subjective reasons, and in this case the listener doesn't notice the fact in advance.

머리가 아파서 약을 먹는 거야.(o)
(I take a pill because I have a headache - providing that taking pills when you are sick is common sense.)
머리가 아프니까 약을 먹는 거야.(o)
(I take a pill because I have a headache - giving an explanation to the listener why I have to take pills.)

In this regard, 니까 has a tendency to be used in colloquial(spoken) language more frequently than 어서.

Even native Koreans cannot explain the differences easily, and there are numerous articles comparing 어서 with 니까, so don't get into a panic about these grammars :)

Download another explanation pdf file here from the European Association for Korean Language Education if needed.