于是 (thereupon / and so)

"…and so" — one thing led to the next

• and so于是yú shì = "thereupon / and so" — a smooth move from cause to consequence.
• to rain下雨xià yǔ(completed / change)了le，and so于是yú shìwe / us我们wǒ mento return home回家huí jiā(completed / change)了le = "it rained, so we went home."
• It chains events in a narrative.

… ，and so于是yú shì …

• and so于是yú shì links a reaction that naturally followed.

rain下雨xià yǔ了le，and so于是yú shìwe我们wǒ mengo home回家huí jiā了le。

• to rain下雨xià yǔ(completed / change)了le，and so于是yú shìwe / us我们wǒ mento return home回家huí jiā(completed / change)了le = "it rained, so we went home."

For events, not logic

• and so于是yú shì chains what happened next; so / therefore所以suǒ yǐ gives a logical "therefore".

Recap

Read the full reference: HSK 4 grammar — Conjunctions.