A point in time before an event or action
When to use
temporal order, before past event, time passage expression
The nuance
Used to indicate time, meaning before a specific past moment or event.
전에 (jeon-e, jeone) · 앞에 (ap-e, ape)
'전에' indicates a point in time before an event or action, while '앞에' refers to a spatial position in front of an object.

One lane per word — no nested boxes, just the gist.
A point in time before an event or action
When to use
temporal order, before past event, time passage expression
The nuance
Used to indicate time, meaning before a specific past moment or event.
Spatial position in front of an object
When to use
spatial location, physical direction, place indication
The nuance
Used to indicate space, referring to the front side of an object or direction.
수업 전에 숙제를 해야 해요.
su-eop jeon-e suk-je-reul hae-ya hae-yo., su eop jeon e suk je reul hae ya hae yo.
Indicates a point in time before class, so '전에' is appropriate.
수업 앞에 숙제를 해야 해요.
su-eop ap-e suk-je-reul hae-ya hae-yo., su eop ap e suk je reul hae ya hae yo.
'앞에' refers to spatial position and cannot be used for time expressions.
학교 앞에 버스 정류장이 있어요.
hak-gyo ap-e beo-seu jeong-ryu-jang-i iss-eo-yo., hak gyo ap e beo seu jeong ryu jang i iss eo yo.
Indicates a spatial location in front of the school, so '앞에' is correct.
학교 전에 버스 정류장이 있어요.
hak-gyo jeon-e beo-seu jeong-ryu-jang-i iss-eo-yo., hak gyo jeon e beo seu jeong ryu jang i iss eo yo.
'전에' is temporal and inappropriate for spatial location sentences.
회의 전에 자료를 준비하세요.
hoe-ui jeon-e ja-ryo-reul jun-bi-ha-se-yo., hoe ui jeon e ja ryo reul jun bi ha se yo.
Indicates time before the meeting starts, so '전에' is used.
회의 앞에 자료를 준비하세요.
hoe-ui ap-e ja-ryo-reul jun-bi-ha-se-yo., hoe ui ap e ja ryo reul jun bi ha se yo.
Not referring to a spatial front of the meeting place or object, so incorrect.
집 앞에 큰 나무가 있어요.
jip ap-e keun na-mu-ga iss-eo-yo., jip ap e keun na mu ga iss eo yo.
Indicates spatial location in front of the house, so '앞에' is appropriate.
집 전에 큰 나무가 있어요.
jip jeon-e keun na-mu-ga iss-eo-yo., jip jeon e keun na mu ga iss eo yo.
Not suitable for spatial location because '전에' is temporal.
Which is correct for time expression?
Which sentence indicates spatial location?
Which is correct: '수업 전에' or '수업 앞에'?