When to use assim vs por isso

Both assim and por isso seems to translate to thus, therefore, so. I wonder whether they are they completely interchangeable, or is there perhaps a subtle difference between them that I just haven’t grasped yet. Is anyone able to help me understand?

@Tomas, sorry for keeping you waiting! When used in a conclusive sense (as thus/therefore/so), I would consider them interchangeable without any greater subtleties, apart from assim sounding more formal than por isso.

  • O mundo mudou. Assim, temos de mudar também. = O mundo mudou. Por isso, temos de mudar também. (The world has changed. Thus, we need to change as well.)

In other uses, they’re not interchangeable. Some examples:

  • Está frio, por isso é que tenho um casaco. (It’s cold, that’s why I have a jacket.)
  • É assim que fazemos. (That’s how we do it.)
  • Um projeto assim é muito ambicioso. (A project like that is very ambitious.)
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