This is the 525th most frequent Serbian word.
отворио
"Opened" (past tense of "open").
Here, the word 'отворио' is used as the past tense of 'отварати' and refers to the action of opening the book that has been completed by a male person.
Он је отворио књигу.
He opened the book.
In this sentence, the word 'отворио' is used to describe the action that the wind performed causatively on the window, depicted in the past tense.
Ветар је отворио прозор.
The wind opened the window.
Here, 'отворио' is used to express the action performed by the speaker (male), indicating the completion of the act of opening a document in a program.