Quick answer
Свой means “belonging to the subject of this sentence”. Он взял свою́ кни́гу = his own book. Он взял его́ кни́гу = someone else’s book.
The forms
| Masculine | Feminine | Neuter | Plural | |
|---|---|---|---|---|
| свой | свой | своя́ | своё | свои́ |
It declines exactly like мой — the difficulty is not the form, it is knowing when to use it.
The difference in practice
The rule in one line
If the owner is the subject of the sentence, use свой. With я and ты it is optional and both sound fine; with он, она́ and они́ it changes the meaning, so it is not optional at all.
Where свой cannot go
Свой can never be the subject itself. “My brother works” is Мой брат рабо́тает — never “свой брат работает”.
Typical mistakes
Он лю́бит его́ жену́ — meaning his own
Он лю́бит свою́ жену́.
With a third person, его́ points at somebody else. This is the classic misunderstanding.
Свой брат рабо́тает в ба́нке.
Мой брат рабо́тает в ба́нке.
свой cannot be part of the subject.
Check yourself
How do you say “He is reading his own book”?