I don't understand when I can use passive voice.
I think you can use it whenever you want. But it doesn't mean you should.
As far as I know, you should use Passive Voice when...
- you don't know who did something. E.g. Oh no! My bag has been stolen!
- who did something is not as important as what happened: E.g. Our product was tested using the most rigorous standards.
- who did the action is obvious from the situation: E.g. My computer is being repaired today.
- a general group of peopke do the action: E.g. New Year's Day is celebrated here on January 1st.