于是: so, thereupon
连词 / conjunctions
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Event 1，于是 + (S) + event 2
于是 links two past actions: the second is the natural response to the first, "so", "thereupon", "and so".
It opens the second clause (before the subject) and is mainly used in narration. It isn't used for general logical conclusions (→ 所以) or suggestions (→ 那就).
Examples
他觉得这个工作没意思，于是辞职了。
Tā juéde zhège gōngzuò méi yìsi, yúshì cízhí le.He found the job boring, so he quit.
视频在网上火了，于是很多人来这里旅游。
Shìpín zài wǎng shang huǒ le, yúshì hěn duō rén lái zhèlǐ lǚyóu.The video went viral, and so lots of people came here as tourists.
天黑了，于是我们决定回家。
Tiān hēi le, yúshì wǒmen juédìng huí jiā.It got dark, so we decided to head home.
Common mistakes
✗ 明天下雨，于是你带伞吧。 → 于是 isn't for future advice → ✓ 所以你带伞吧 / 那你带伞吧。
✗ 因为…于是… → ✓ 因为…所以…; 于是 stands alone.