In English, "Lars is trimming his hedge" is ambiguous: is it his own hedge, or another man's? Swedish eliminates this confusion using the reflexive possessives sin, sitt, and sina.
The Subject Rule
Whenever a third-person subject (he, she, it, they) owns an object in the same clause, use a reflexive possessive to mean "his own," "her own," "its own," or "their own." For en words, use sin.
Contrast: His Own vs. Someone Else's
If you use standard third-person possessives (hans, hennes, dess, deras) for an object, they cannot refer to the subject. They always mean someone else. Notice how sin points back to the subject's own property, while hans and hennes point to the neighbor's:
Agreement: Sitt and Sina
The reflexive pronoun must agree in gender and number with the noun being owned, not the owner. Use sin for en words, sitt for ett words, and sina for plurals.