(d) The father spoke to his (sia) son and his (own) (siaj) friends: he spoke also to his (the son’s) (liaj) friends. After the death of my aunt, my mother educated her children with her own. Mr. Verd invited his friend and his (the friend’s) family. Rose walked with Lily and with her (own) brother; also with her (Lily’s) brother. Henry opened his window, and saw his sweetheart, who opened her window at the same time. My friends love their children. I also love their children. My friend went with his wife to their country house, to visit her parents and their (the parents’) relations and his uncle.
838. The father’s pen is in his pocket = La plumo de la patro estas en lia poŝo. Not sia, which would mean in its own pocket! (Rule II), because the subject is plumo. Petro kantis kun Gertrudo kaj ŝia kuzo, Peter sang with Gertrude and her cousin. (Here sia would mean his own cousin (Rule II).)
She went with Robert and his mother. I walked with my dog and its pup. He will visit my neighbour and her daughter. We met Charles and Thomas with their friend.
839. In the sentence Petro kantis kun sia edzino we use sia, not lia, because the pronoun refers to the subject Petro (Rule II). But in Lia edzino kantis kun li we use lia, not sia, because lia edzino is the subject (Rule I).
840. John greets his friend (Johano salutas sian amikon—Rule II). His friend (Lia amiko—Rule I) greets him. The child learns its lesson. Its lesson is easy. The mother loves her daughter, but her daughter does not love her. The ladies left their house, because their house was too small. The father wrote to his son, and his son replied. The child lost its doll. I showed (to) the child its doll.
841. In the sentence Petro kaj lia edzino kantis, however, we use lia, not sia, because the subject is Petro-kaj-lia-edzino. It is really two sentences packed into one: (a) Petro kantis, and (b) Lia edzino kantis; and sia cannot be the subject (Rule I). Similarly: Petro kantis, sed ne lia edzino (=sed lia edzino ne kantis).
N.B. A conjunction (e.g., kaj, sed) introduces a new sentence: a preposition (e.g., kun in 839) does not.
842. (a) Si relates to the whole subject, not to a part of the subject only. Ŝi-kaj-li vizitas sian (their) avon. Petro-kaj-lia-edzino kantis kun lia (his) patro kaj ŝia (her) patrino kaj sia (their common) amiko kaj ties (the friend’s) kuzino.
843. (a) My friend and his brother love their sister. My friend goes with his brother and helps their sister.
(b) Translate the four meanings of P told J that he would have to leave (devos lasi) his house, translating he by li to denote P, and by tiu to denote J.
(c) The advanced student may find out and translate the forty possible meanings of When P met J, he was with his friend and his cousin, using ties (426) to translate the latter’s (the friend’s) cousin (1191).
844. Use si only when you are sure that is in order. It is a bad mistake to use si when it should not be used. And even where si is correct, there are cases where it may be clearer not to use it. When in doubt, leave it out.