In many cases, they are equal, but there are still some differences. Here are some examples in which these words can't be replaced by each other: 1. 吸烟对身体不好，所以你应该戒烟。 Smoking is not good for health, so you should quit smoking. 2. 我递给他一支烟，于是他抽了起来。 I gave him a cigarette, and he smoked. 于是 is more particular used to describe something, in a temporal sequence 所以 is mostly used to prove something, focusing on the result of a reason

我举个例子: 她的丈夫去世了，所以她很伤心，于是她不吃饭也不说话。("于是"强调最终的结果)

Generally, no diffirence. But we use 所以 more, and 于是 is mainly used in articles.

所以—帶有因果關係，前面要有原因( reason...,所以+outcome) 於是—接續前面的句子(承接前意)，後面加上the following events 所以(B)和於是(A)的關係 A ⊆ B

大家谢谢！