於是

1. Narrative Function
於是 (jyu1 si6) is used when recounting a series of events. It signals that 'Event A happened, and as a result of that, Event B followed.' It is very common in storytelling or explaining a sequence of personal decisions.

2. Narrative Sequence vs. Logical Deduction
Unlike 所以 (so2 ji5), which can describe any logical cause-and-effect or static deduction, 於是 focuses on the sequential flow of events in a narrative. It tells us what happened next as a result of the previous event. Learners often confuse 於是 with 然後 (jin4 hau6). A helpful way to remember the difference is this triad: 所以 indicates Consequence, 然後 indicates Sequence, and 於是 indicates Sequence + Consequence. In past-tense narratives where an event is both sequential and causal, 於是 and 所以 are virtually interchangeable.

3. Tense and Subject Placement
於是 is almost exclusively used for past or completed events in a narrative; it is not used for future predictions or hypothetical 'if' statements. In spoken Cantonese, learners should stick to placing the conjunction at the very beginning of the clause, before the subject (e.g., 於是乎我... jyu1 si6 fu4 ngo5). While placing the subject before the conjunction (e.g., 我於是... ngo5 jyu1 si6) is grammatically correct in Standard Written Chinese, it sounds highly unnatural and bookish in spoken Cantonese. If the subject remains the same across both clauses, you can either retain the subject pronoun in the second clause (e.g., 於是我就...) or drop it entirely—both are extremely common and natural.

4. Spoken vs. Written
While 於是 is common in written Chinese, it is also used in speech. However, 於是 alone can sound slightly bookish or formal in spoken Cantonese. To soften it and ground it as a natural colloquial storytelling device, speakers often add the particle 乎 (fu4) to make it 於是乎 (jyu1 si6 fu4). In very casual conversation, speakers often prefer 噉 (gam2) to mean 'so/then,' which is frequently expanded to 噉樣 (gam2 joeng2) or 噉就 (gam2 zau6) at the start of the clause. Note that while 噉就 overlaps with 於是 in storytelling, 噉就 (gam2 zau6) can also mean 'in that case' for conditional or hypothetical situations, whereas 於是 is strictly narrative ('thereupon').

5. Placement and Constraints
It is a free-standing conjunction that appears at the start of the second clause. It is not the usual partner of 因為 (jan1 wai6): 因為...於是... can appear in writing, but 因為...所以... is the plain way to give a reason and its result. Most often 於是 simply follows a clause that describes the first event.

6. Pairing with 就 (zau6)
In spoken and informal written Cantonese, 於是 and 於是乎 are overwhelmingly paired with the adverb 就 (zau6) in the second clause to emphasize the immediate consequence (e.g., 於是我就...).