於是grammar point · tier 2 · consequential 於是 (and so, thereupon)
Connective opening the second clause: given the situation just stated, here is the action it led to (and so, thereupon).
字源 FORM what the parts do
於 is a worn variant of 烏 the crow, hired out as the joint word at, in, to; it carries no connective meaning of its own. 是 is the classical pointer, this. Set together they read upon-this: thereupon.
故事 STORY a scene to remember it by
A situation settles flat on the ground; 於是 plants a foot on it and steps forward into the next move.
字源記憶法
框 · Frame
[situation] ，於是 [subject] [resulting action]
觸 · Trigger
A situation has been described and you want to swing into the action it set off.
序 · The move
1State the situation as a full first clause and close it.Is the trigger already on the page before 於是 appears?
2Open the second clause with 於是, then name the action that followed.Is the second clause a deliberate action or decision, not just an outcome that happened on its own?
3Confirm the order cannot flip.Does the action depend on the situation, so swapping the clauses breaks the sense?
例 · Examples
1抽煙to smoke有害harmful to身體健康，於是and so, thereupon爸爸決定to decide戒煙to quit smoking。
Smoking harms your health, and so Dad decided to quit.
因為…所以… lays out the logic of why one thing causes another; 於是 pivots from a situation into the next action, with no claim about reasoning. 因為 can open its sentence; 於是 never starts the chain, it only continues it.
結果 marks an outcome that turned out a certain way; 於是 marks a deliberate response someone took. 於是 leans toward a chosen action, 結果 toward a result that landed.
✗ 於是抽煙有害身體健康，爸爸決定戒煙。 → ✓ 抽煙有害身體健康，於是爸爸決定戒煙。 (於是 cannot open the chain; the situation comes first)
✗ 因為下雨，於是所以我們回家了。 → ✓ 因為下雨，所以我們回家了。 / 下雨了，於是我們回家了。 (don't stack 於是 with 所以 in the same clause)
✗ 他很努力，於是很聰明。 → ✓ 他很努力，於是進步了。 (於是 needs a following action or change, not a static trait)
English 'so' covers both reasoning ('so it follows that…') and sequence ('and so I left'); 於是 only does the sequence-into-action sense, so reaching for it wherever English says 'so' overuses it where 因此/所以 is wanted.