Use si or sia when referring to the already-introduced subject of the sentence:

• La virino rigardis la bildon antaŭ si -> "The woman looked at the picture in front of her", the picture is in front of the subject of the sentence ("the woman")
• Li eniris sian domon -> "He entered his (own) house", the house belongs to the subject of the sentence ("he")
• La infano amas sian patrino -> "The child loves its (own) mother", the mother belongs to the subject of the sentence ("the child")

Use si or sia when referring to the already-introduced subject of the sentence:

• La virino rigardis la bildon antaŭ si -> "The woman looked at the picture in front of her", the picture is in front of the subject of the sentence ("the woman")
• Li eniris sian domon -> "He entered his (own) house", the house belongs to the subject of the sentence ("he")
• La infano amas sian patrinon -> "The child loves its (own) mother", the mother belongs to the subject of the sentence ("the child")

Use si or sia when referring to the already-introduced subject of the sentence:

• La virino rigardis la bildon antaŭ si -> "The woman looked at the picture in front of her", the picture is in front of the subject of the sentence ("the woman")
• Li eniris sian domon -> "He entered his (own) house", the house belongs to the subject of the sentence ("he")
• *La infano amas sian patrino" -> "The child loves its (own) mother", the mother belongs to the subject of the sentence ("the child")

Use si or sia when referring to the already-introduced subject of the sentence:

• La virino rigardis la bildon antaŭ si -> "The woman looked at the picture in front of her", the picture is in front of the subject of the sentence ("the woman")
• Li eniris sian domon -> "He entered his (own) house", the house belongs to the subject of the sentence ("he")
• La infano amas sian patrino -> "The child loves its (own) mother", the mother belongs to the subject of the sentence ("the child")