Giving Reasons: -아서/어서 vs. -(으)니까

Explanation

-아서/어서 links a general cause and its result, and it is the form used with set expressions like 고맙다, 미안하다, and 반갑다. You cannot put the past marker -았/었- before -아서/어서, and it cannot be followed by a command or a suggestion. -(으)니까 gives a reason based on the speaker's own judgment and is the one to use before commands and suggestions such as -(으)세요, -(으)ㅂ시다, and -(으)ㄹ까요?. Unlike -아서/어서, it can follow -았/었-.

How to form it

• Stem + 아서/어서 (하다 → 해서)

  늦다 → 늦어서, 피곤하다 → 피곤해서

• Stem ending in a consonant + 으니까; stem ending in a vowel + 니까

  없다 → 없으니까, 오다 → 오니까

• Past reason: stem + 았/었으니까 (-았/었어서 is not used)

  먹다 → 먹었으니까

Examples

• 배가 아파서 병원에 갔어요.

  I went to the hospital because my stomach hurt.

• 늦어서 죄송합니다.

  I'm sorry I'm late.

• 비가 오니까 우산을 가져가세요.

  It's raining, so take an umbrella.

• 시간이 없으니까 택시를 탑시다.

  We don't have time, so let's take a taxi.