What's the difference between 于是 and 所以? How and when are they interchangeable?
there's a blog post with examples about just this question here:
To quote, it says:
所以 introduces the outcome, just as in English where we use ‘therefore’, ‘as a result’ or ‘so’.
... Moving on to 于是. 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, and translates best to ‘thereupon’, ‘as a result’, ‘consequently’.
It could be helpful to look at them in ancient Chinese.
于=from，是=这=here (the point). So 于是=from here (or conclude from the point).
所以 is not so straight forward. 以=according to/depend on/use something for some purpose/etc. 所 indicate something is passive.
For example AAA所以BBB = BBB depends on AAA/BBB becomes what it is now is due to AAA.
So clearly, 于是 is used to derive. 所以 is used to retrieve the reason.
But in modern Chinese the difference is not so significant. Please read S.Rhee's comment for the details.