于是【yúshì】

于是 starts the second of two events and says the first one prompted it. Both have already happened, and someone usually decided to do the second.

于是 goes at the start of its clause, and a subject can follow it.

于是 or 所以

所以 states a result of a reason, at any time: past, present, future or always true. 于是 is narrower. It moves a story on to the next thing that happened, so it never introduces a plan, a command or a general fact.

• × 明天下雨，于是我们不去了 → 明天下雨，所以我们不去了
• × 他是中国人，于是会说中文 → 他是中国人，所以会说中文

In stories

于是 is at home in written narration and storytelling, linking a chain of events. In a story it often starts a new sentence.