Possession "свой"

свой means "own"; that it belongs to the subject of the clause. The form of свой must agree with the gender of the noun.

своя книга (f.), свой журнал (m.), свои дети (pl.), своё пальто (n.)

Even though in English there is only 'his', here you cannot mix свой and его, look at this example.

• иван думал, что виктор нашёл свою книгу
  • ivan thought that viktor found viktor's own book.
• иван думал, что виктор нашёл его книгу
  • ivan thought that viktor found his (ivan's) book.

Couple of notes

In the case of я, мы, вы, the use of свой is optional since it is the same as using мой, наш, ваш.

For ты, свой is preferred to твой.

• ты будешь пить свой (not твой) напиток?
  • Are you going to drink your drink?

You can make your life easier by remembering that Russians often do not use and equivalent of 'my', 'his', etc. if the possessor is obvious.

• возьмите пальто
  • take your coat (no need for свой)