To show a smile or laughter because of happiness or joy
When to use
When happy, Reacting to a joke, Feeling good
The nuance
Use when expressing positive or joyful emotions. Not related to sadness or pain.
웃다 (us-da, usda) · 울다 (ul-da, ulda)
웃다 (ut-da) and 울다 (ul-da) express opposite emotions: laughter and crying.

One lane per word — no nested boxes, just the gist.
To show a smile or laughter because of happiness or joy
When to use
When happy, Reacting to a joke, Feeling good
The nuance
Use when expressing positive or joyful emotions. Not related to sadness or pain.
To shed tears and make sounds due to sadness or pain
When to use
When sad, When in pain, Tears from emotion
The nuance
Use when expressing negative or painful emotions. Not related to happiness or joy.
그녀가 농담을 듣고 크게 웃었다.
geu-nyeo-ga nong-dam-eul deut-go keu-ge us-eoss-da., geu nyeo ga nong dam eul deut go keu ge us eoss da.
웃다 is appropriate to express joy here.
그는 슬픈 영화를 보고 웃었다.
geu-neun seul-peun yeong-hwa-reul bo-go us-eoss-da., geu neun seul peun yeong hwa reul bo go us eoss da.
Laughing at a sad situation is inappropriate; 울다 should be used.
아기가 배가 아파서 울고 있다.
a-gi-ga bae-ga a-pa-seo ul-go iss-da., a gi ga bae ga a pa seo ul go iss da.
울다 fits expressing pain.
아기가 배가 아파서 웃고 있다.
a-gi-ga bae-ga a-pa-seo us-go iss-da., a gi ga bae ga a pa seo us go iss da.
Laughing due to stomach pain is illogical; 울다 is correct.
친구와 만나서 기뻐서 웃었다.
chin-gu-wa man-na-seo gi-ppeo-seo us-eoss-da., chin gu wa man na seo gi ppeo seo us eoss da.
Appropriate to express joy with 웃다.
친구와 만나서 기뻐서 울었다.
chin-gu-wa man-na-seo gi-ppeo-seo ul-eoss-da., chin gu wa man na seo gi ppeo seo ul eoss da.
Using 울다 for joy is unnatural.
감동적인 연설을 듣고 많은 사람이 울었다.
gam-dong-jeok-in yeon-seol-eul deut-go man-eun sa-ram-i ul-eoss-da., gam dong jeok in yeon seol eul deut go man eun sa ram i ul eoss da.
Tears from emotion are expressed with 울다.
감동적인 연설을 듣고 많은 사람이 웃었다.
gam-dong-jeok-in yeon-seol-eul deut-go man-eun sa-ram-i us-eoss-da., gam dong jeok in yeon seol eul deut go man eun sa ram i us eoss da.
Laughing at an emotional speech is inappropriate; 울다 is correct.
Which verb should you use when watching a sad movie?
I hear a funny joke from a friend, then I?
What do you do when you are in pain?