to stop movement or an action
When to use
vehicle suddenly stops, stopping while walking, machine stops working
The nuance
'멈추다' is used when a movement or action that was ongoing comes to a halt. It emphasizes the cessation of action.
멈추다 (meom-chu-da, meomchuda) · 서다 (seo-da, seoda)
'멈추다' means to stop an action or movement, while '서다' means to stand or be in an upright position at a place.

One lane per word — no nested boxes, just the gist.
to stop movement or an action
When to use
vehicle suddenly stops, stopping while walking, machine stops working
The nuance
'멈추다' is used when a movement or action that was ongoing comes to a halt. It emphasizes the cessation of action.
to stand upright at a place
When to use
standing by the roadside, people lining up, standing at a bus stop
The nuance
'서다' indicates a person or object standing upright at a certain place, focusing on position and posture rather than the act of stopping movement.
차가 갑자기 멈췄다.
cha-ga gap-ja-gi meom-chwoss-da., cha ga gap ja gi meom chwoss da.
The movement stopped, so '멈추다' is correct.
차가 갑자기 섰다.
cha-ga gap-ja-gi seoss-da., cha ga gap ja gi seoss da.
'서다' is usually used for people standing, so it's awkward for a vehicle stopping; use '멈추다' instead.
사람들이 버스 정류장에 서 있다.
sa-ram-deul-i beo-seu jeong-ryu-jang-e seo iss-da., sa ram deul i beo seu jeong ryu jang e seo iss da.
People standing at a place, so '서다' is appropriate.
사람들이 버스 정류장에 멈춰 있다.
sa-ram-deul-i beo-seu jeong-ryu-jang-e meom-chwo iss-da., sa ram deul i beo seu jeong ryu jang e meom chwo iss da.
'멈추다' focuses on stopping an action, so it's inappropriate for indicating location; use '서다'.
걷다가 갑자기 멈췄다.
geot-da-ga gap-ja-gi meom-chwoss-da., geot da ga gap ja gi meom chwoss da.
Stopped walking, so '멈추다' is correct.
걷다가 갑자기 섰다.
geot-da-ga gap-ja-gi seoss-da., geot da ga gap ja gi seoss da.
Stopped walking and now standing is natural; '서다' is possible, but '멈추다' better emphasizes stopping action.
기계가 작동을 멈췄다.
gi-gye-ga jak-dong-eul meom-chwoss-da., gi gye ga jak dong eul meom chwoss da.
Machine operation stopped, so '멈추다' is correct.
기계가 작동을 섰다.
gi-gye-ga jak-dong-eul seoss-da., gi gye ga jak dong eul seoss da.
'서다' usually means standing upright, so it's inappropriate for machine operation stopping; use '멈추다'.
The car suddenly ___.
People are ___ by the roadside.
Suddenly ___ while walking.
The machine stopped operating.