Why "sia domo" rather than "ilia domo"?
When the possessor in the object position is the same entity as the subject of the sentence, you use the possessive "sia". http://en.wiktionary.org/wiki/sia#Esperanto
Makes sense - many thanks!
How then would "they have good tools in her house" be said in esperanto.
That would be "Ili havas bonajn ilojn en ŝia domo." Note the accent on the "ŝ". "Ŝia" is the possessive "her", while "sia" is what Luis_Domingos described above.
Is there a plural form of this word? “Siaj”
I think there's a potential ambiguity in your question. Could you be more precise?
From another comment: http://en.wiktionary.org/wiki/sia#Esperanto:
sia (accusative singular sian, plural siaj, accusative plural siajn)
But only for the third person singular and plural. 'Mi' and 'vi' requires mia(j)(n) and via(j)(n).
I gave "It's understandable they have good tools in their house." instead of "Understandably ..." or "Of course ..." Am I that far wrong?
I'm wondering the same thing. I put "It's understandable hat they have good tools in their house."
Kombreneble is an adverb, therefore "understandably" is a closer translation.
I put 'understandably' and it was marked wrong, which I'm thinking I should report.
Would this sentence not require that Sia be pluralised ? Ilia - siaj ?
No, because sia complements domo: sia domo = their house.
Thanks. I got it now. :)