所以 causal relationship
 Used as correlative conjunctions
 所以 is often partner with 因为
 reason所以+outcome
1. 因为他妈妈是法国人，所以他会说法语。
Cause / Reason effect/Outcome
2. 他不太舒服，所以不想吃东西。
Cause / Reason effect/Outcome
于是 sequential relationship (it has a meaning “then”)
 This is used to place emphasis on the chronology of two connected happenings:
A –> B, in continuation. Often it’s used in recounting past events or a story.
 The second clause is a new situation or action caused by the first clause.
However, if there is a discernible causal relationship, 于是 can be replaced by
所以。
1. 他很喜欢这件毛衣，于是就买下来了。
Then I bought.
= 他很喜欢这件毛衣，所以就买下来了。
Cause/reason effect/outcome
2. 下雨了，于是我们不去踢足球了。
Then we don’t go to play soccer
= 下雨了，所以我们不去踢足球了。
Cause/reason effect/outcome
As the above examples illustrate, 于是 introduces the next thing to happen in
sequence. There is some degree of interchangeability with these words: you can use
所以 in place of 于是 most of the time, but not often the other way around.
1. 他想学跳舞，我也想，于是我们一起去上跳舞课。
Then we go to the dancing class together
=他想学跳舞，我也想，所以我们一起去上跳舞课。
Cause/reason effect/outcome
2. 今天下雪，我又忘了带外套，所以觉得特别冷。
Cause/reason effect/outcome
3. 上海人多车多，所以开车容易紧张。
Cause/reason effect/outcome