I feel like I kind of know this, but sometimes I feel unsure. It’s pretty similar to the Swedish sin, sitt. Is there a good rule or a nice, short explanation for when and where to use si, sia instead of li, sxi?
You asked for short:
Use si/sia when both are true
Regarding point 1: you'll never see la viro kaj sia edzino estas... because lia edzino is part of the subject.
Regarding point 2: it's important to notice that it's the subject of the verb and not the subject of the sentence, because while they're usually the same, they are not always the same.
Answered by Tomaso Alexander on December 4, 2021
The grammar at Lernu regarding pronouns has detailed information about the use of si and sia, available in many languages (the link is to the English version), that is too long to copy here with all the examples. It is worth checking it out.
Answered by Lyubomir Vasilev on December 4, 2021
Use si or sia when referring to the already-introduced subject of the sentence:
Answered by Chives on December 4, 2021
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