於 — Formal preposition fixing the time or place at which an action happens; slots in front of the verb.

TSUMUGU · TBCL 5 (est.) · 語法

於 grammar point · tier 1 · 於 — formal preposition marking time/place of an event

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Formal preposition fixing the time or place at which an action happens; slots in front of the verb.

字源 FORM what the parts do

The same bird as 烏 the crow, one graph worn into two; this one was hired out as an empty joint of the sentence and carries no picture of its own. It clamps a time or a place onto the verb that follows. Written register; speech reaches for 在 instead.

故事 STORY a scene to remember it by

A bolt set just ahead of the action, and the moment of the event locks onto it.

字源記憶法

框 · Frame

[subj] (已/將) 於 [time / place] [verb phrase]

觸 · Trigger

Pin a formal account of an event to the exact hour or site it happened at, before naming the action.

序 · The move

1Name the event and put its subject first.Is this a written or formal register? In speech, switch to 在.

2Drop 於 in front of the verb, then the time or place right after it.Does 於 sit before the verb, not after the object?

3Run the verb phrase last.Could the spoken 在 (在3 for a time, the locative 在 for a place) replace 於 in the same slot? If yes, the slot is right.

例 · Examples

1總經理general manager已already於at (formal)下午兩點two in the afternoon前往set out for越南拜訪pay a visit to客戶client。

The general manager already set out at two in the afternoon to visit clients in Vietnam.

界 · Boundary

在3

於 and 在 both front a time onto the verb (於下午兩點 / 在下午兩點); 於 is the written, formal joint and reaches for an exact point, 在 is the everyday spoken default. For a place, 於 pairs the same way with the locative 在 (於台北 / 在台北). Same slot, register split.

向

於 fixes WHEN or WHERE the action sits; 向 aims the action at a target (向他借錢). Anchor against direction.

✗ 他出發於兩點。 → ✓ 他於兩點出發。 (於 + time goes before the verb, not trailing after it)

✗ 我們昨天於餐廳吃飯。 → ✓ 我們昨天在餐廳吃飯。 (everyday speech takes 在, not 於)

✗ 會議於明天。 → ✓ 會議於明天舉行。 (the 於-phrase needs the verb it clamps onto; it cannot end the sentence bare)

English 'at 2 pm' floats to the sentence end; 於 forces the time in front of the verb. English 'at' has no spoken-versus-written split, so learners overuse 於 where 在 belongs.