In short

Korean puts the time word after the clause it frames, and each time word demands one specific verb form: -기 전에 (before), -(으)ㄴ 후에 (after), -(으)ㄹ 때 (when), -자마자 (as soon as), -(으)면서 (while, one person doing two things at once), and -는 동안 (during). Match the form to the word, and keep the space before every one of those time nouns (전, 후, 때, 지, 동안).

The rule

Step 1 - Before: -기 전에 (tenseless)

Add the nominalizer -기 to a bare verb stem, then the noun 전 (jeon, before) plus 에: Verb stem + -기 전에. The stem stays bare - no tense, no 받침 choice, same for every verb: 자다 -> 자기 전에 (jagi jeone, before sleeping), 먹다 -> 먹기 전에 (meokgi jeone, before eating). With a plain noun, use Noun + 전에: 식사 전에 (siksa jeone, before the meal).

Step 2 - After: -(으)ㄴ 후에 (past modifier)

“After” takes the past modifier -(으)ㄴ (lesson 24) on the stem, then 후에: 먹은 후에 (meogeun hue, after eating), 간 후에 (gan hue, after going). 후 freely swaps for 다음 (daeum) or 뒤 (dwi): 먹은 다음에 (meogeun daeume), 먹은 뒤에 (meogeun dwie) - all the same meaning. With a noun: 수업 후에 (sueop hue, after class).

Step 3 - When: -(으)ㄹ 때 vs -았/었을 때

“When / while” takes the prospective modifier -(으)ㄹ on the stem, then 때 (ttae, time): 갈 때 (gal ttae, when going), 먹을 때 (meogeul ttae, when eating), 어릴 때 (eoril ttae, when young). This is the general or ongoing “at the time of.” To mark that the event was already completed at that moment, add the past -았/었- first: 갔을 때 (gasseul ttae, when [I] had gone / once there). The contrast is real: 미국에 갈 때 = on the way to the US, 미국에 갔을 때 = once I was already there. With a noun: 방학 때 (banghak ttae, during vacation). An honorific -(으)시- (lesson 22) just lengthens the stem, and the time word then attaches to that longer stem exactly as it would to a plain one: 읽으실 때 (ilgeusil ttae, when [she] reads), 가시기 전에 (gasigi jeone, before [he] goes), 계시는 동안 (gyesineun dongan, while [she] is here) - 계시다 is the honorific of 있다, so it takes the verb-style -는.

Step 4 - Since: -(으)ㄴ 지 + duration + 되다/지나다

To say how much time has passed since something started, use the past modifier -(으)ㄴ + the bound noun 지 + a duration + 되다 (to become) or 지나다 (to pass): 한국에 온 지 3년 됐어요 (Hanguge on ji sam nyeon dwaesseoyo, It’s been three years since I came to Korea). Here 지 means elapsed time since, not the same 지 as -지 않다. Keep the space: 온 지, never *온지.

Step 5 - As soon as: -자마자 (bare stem, no space)

Attach -자마자 straight onto the bare verb stem - no 받침 choice, no tense, no space: 도착하자마자 전화했어요 (Dochakajamaja jeonhwahaesseoyo, As soon as I arrived, I called), 먹자마자 (meokjamaja, as soon as [I] eat). It means the second event follows the first with no gap.

Step 6 - While (same subject): -(으)면서

-(으)면서 joins two actions the same person does at the same time: -면서 after a vowel or ㄹ, -으면서 after a 받침. It carries no tense (the main clause does that) and the subject must be shared: 음악을 들으면서 공부해요 (Eumageul deureumyeonseo gongbuhaeyo, I study while listening to music). Its concessive cousin -(으)면서(도) means even though, and it is the one use that needs no second action: 알면서도 모르는 척해요 (Almyeonseodo moreuneun cheokaeyo, (she) pretends not to know even though she does) - and on a descriptive verb it simply sets two properties of one thing side by side, with no agent and nothing for two clauses to share: 이 방은 좁으면서도 밝아요 (I bangeun jobeumyeonseodo balgayo, this room is small yet bright).

Step 7 - During (any subject): -는 동안(에)

For a stretch of time, add the present modifier -는 + the noun 동안 (dongan, during/while): 먹는 동안 (meongneun dongan, while eating). Unlike -(으)면서, the two clauses may have different subjects: 제가 자는 동안 동생이 왔어요 (Jega janeun dongan dongsaengi wasseoyo, while I was sleeping, my younger sibling came). With a noun: 방학 동안 (banghak dongan, during vacation), 3년 동안 (sam nyeon dongan, for three years).

Step 8 - Watch the ㄹ-stems

A ㄹ-stem behaves differently with each ending. Before -(으)ㄹ 때 the two ㄹ merge to one: 살 때 (sal ttae). Before the ㄴ of -(으)ㄴ 후에 and the ㄴ of -는 동안 the ㄹ drops: 산 후에 (san hue), 사는 동안 (saneun dongan), 만든 후에 (mandeun hue). But before -(으)면서 and -기 the ㄹ stays: 살면서 (salmyeonseo), 살기 전에 (salgi jeone).

Forms & patterns

The master table - which form each time word demands:

-(으)면서 - the one ending with a real allomorph split:

One ㄹ-stem, five patterns - the ㄹ does something different each time:

Examples

Common mistakes

• 텔레비전을 봤으면서 먹어요. (Tellebijeoneul bwasseumyeonseo meogeoyo.) -> 텔레비전을 보면서 먹어요. (Tellebijeoneul bomyeonseo meogeoyo.) - in its simultaneous “while” use, -(으)면서 stays tenseless; only the main verb 먹어요 carries the tense. I eat while watching TV.
• 밥을 먹은 전에. (Babeul meogeun jeone.) -> 밥을 먹기 전에. (Babeul meokgi jeone.) - 전에 pairs with the nominalizer -기, never the past modifier -(으)ㄴ. Before eating.
• 살은 후에. (Sareun hue.) -> 산 후에. (San hue.) - 살다 is a ㄹ-stem, so the ㄹ drops before the ㄴ of -(으)ㄴ, giving 산 - not *살은. (Contrast 살 때, where the two ㄹ merge instead.)
• 미국에 갈 때 영어를 배웠어요. (Miguge gal ttae yeongeoreul baewosseoyo.) -> 미국에 갔을 때 영어를 배웠어요. (Miguge gasseul ttae yeongeoreul baewosseoyo.) - to mean once I was already there, the event is completed, so use -았/었을 때; 갈 때 means on the way there.
• 도착했자마자 전화했어요. (Dochakaetjamaja jeonhwahaesseoyo.) -> 도착하자마자 전화했어요. (Dochakajamaja jeonhwahaesseoyo.) - -자마자 attaches to the bare stem, never to a tensed -았/었- stem. As soon as I arrived, I called.

Tips

• Before = 기, After = ㄴ, When = ㄹ. 전 (before) looks forward, so it takes the plain -기; 후·다음·뒤 (after) look back, so they take the past -(으)ㄴ; 때 (when) takes the prospective -(으)ㄹ. Memorize the three pairings and most of the lesson is done.
• -(으)면서 = one person; -는 동안 = one span. If the two clauses share a subject doing two things at once, use -(으)면서. If the subjects differ, or you just mean “during that stretch,” you must switch to -는 동안.
• -자마자 is instant, -(으)ㄴ 후에 is merely later. 도착하자마자 전화했어요 = the call came the instant I arrived; 도착한 후에 전화했어요 = I called at some point after arriving. Reach for 자마자 only when there is no gap.
• If a noun can sit in front of it, it takes a space. 전, 후, 다음, 뒤, 때, 동안 are ordinary nouns and can follow a plain noun too (식사 전에, 방학 때, 3년 동안); only 지 is a true bound noun (의존명사) that never stands alone. Either way they are nouns, so they always get a space - unlike the endings -기, -자마자, -(으)면서, which fuse onto the stem.