The passive voice is used when the agent (the person doing the action) is not important for the sentence. The opposite, the "normal" form is called the active voice.

All passives use the perfect participle of the verb. For the verb to write it is "written".
This is used with the verb "to be".

Examples of changes from active to passive:

Active: People produce champagne in France.
The word "people" is not relevant so it is better to say:
Passive: Champagne is produced in France.

Active: They sell 10,000 copies of the newspaper every day.
The word "they" doesn't give any real information: it is better to say:
Passive: 10,000 copies of the newspaper are sold every day.

Here are the affirmative, negative and question forms of the passive in another example:

Is petroleum imported from Belgium?
No. Of course petroleum isn't imported from Belgium.
Where is it imported from?
Well, petroleum is imported from many countries. A lot is imported from Saudi Arabia.
So what is imported from Belgium?
Chocolates! Chocolates are imported from Belgium.