138. The following are sentences more complicated, showing the advantage of Esperanto over English, or indeed over most national languages, in respect of its great clearness of expression:—
Petro skribis al Paŭlo, ke li konduku al li sian hundon = Peter wrote to Paul to bring him his dog (sian refers to Paul because li = he (Paul) becomes the subject, and sia always, as we have said, refers to the subject of its own particular clause; if, therefore, lian had been the possessive, it would have been Peter’s dog.)
Post kiam la reĝo mortis, lia filo edziĝis kun sia/lia kuzino = After the king died, his son married his cousin. Here sia would refer to filo, the son’s cousin, but lia to reĝo, the king’s cousin.
Mia patrino petis sian filinon, ke ŝi donu al sia/ŝia fratino sian/ŝian libron. This sentence in English, whichever of the possessives be used, would be "My mother requested her daughter to give (that she give) to her sister her book."
Now the mother is the first subject in this sentence, therefore sian filinon is the mother’s daughter (ŝian would mean someone else’s daughter). But afterwards ŝi (the daughter) becomes the subject, therefore sia fratino would be the daughter’s sister, and ŝia fratino the mother’s. For the remainder of the sentence ŝi (the daughter) still remains the subject, therefore sian libron would be the daughter’s book, and ŝian libron the sister’s or the mother’s book. Ties libron would definitely specify it as the sister’s book.
138 (a). From the examples given in paragraphs 128, 135, 136, and 138, we see that the reflexive pronoun si and the possessive sia always refer to the subject (noun or pronoun) of their own clause.
(b). Si and sia are likewise used when the noun or pronoun to which they refer is not expressed, but only understood.
Example.—Tio estas nova metodo brosi sian (or, la) ĉapelon! = That is a new method of brushing one’s hat!
(c). Phrases introduced by an infinitive or a participle may be regarded as new sentences, the subject of which is mentally supplied: i.e., the infinitive or the participle may be expanded to make the full sentence implied.
Examples of Infinitive.—Estas bone ne trompi (ke oni ne trompu) sin = It is well not to deceive oneself. La patrino lasis Marion legi (ke ŝi legu) sian libron (Mary’s book); ŝian libron (the mother’s book). La oficiro petis Aŭguston rigardi (ke li rigardu) lin (him) sin (himself). La patro petis sian filon, helpi (ke la filo helpu) sian (the son’s) amikon; lian (the father’s) amikon.