So, thereupon: 于是
连词 / conjunctions
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
Event 1，于是 + (Subject) + Event 2
于是 (yúshì) links two past events: the second is the natural reaction to the first. "So, and then, thereupon".
It's typical of storytelling: 天下雨了，于是我们回酒店了 (it started raining, so we went back to the hotel).
Difference from 所以: 所以 expresses a logical cause (any time); 于是 narrates a sequence of actions in the past. It isn't used for general truths.
Examples
火车晚点了，于是我们去吃了点儿东西。
Huǒchē wǎndiǎn le, yúshì wǒmen qù chī le diǎnr dōngxi.The train was delayed, so we went to grab a bite.
他找不到护照，于是马上给大使馆打了电话。
Tā zhǎo bu dào hùzhào, yúshì mǎshàng gěi dàshǐguǎn dǎ le diànhuà.He couldn't find his passport, so he immediately called the embassy.
酒店没有房间了，于是我们换了一家。
Jiǔdiàn méiyǒu fángjiān le, yúshì wǒmen huàn le yì jiā.The hotel was full, so we switched to another one.
Common mistakes
✗ 明天下雨，于是我不去。 → 于是 narrates the past. ✓ 明天下雨，所以我不去。
✗ 因为……于是…… → don't pair 因为 with 于是.