Boken blir skrevet av læreren.

This is the dynamic (eventive) passive formed with the auxiliary verb blir (present tense of bli, “to become”) plus the past participle skrevet (written). It’s used to emphasize the action rather than who does it. The structure is:

• Subject (“boken”) + blir
  • past participle.
    You can translate it literally as “The book becomes written by the teacher,” which in English we render “The book is being written by the teacher.”

Yes. Norwegian offers two dynamic passive forms:

• Boken blir skrevet av læreren. (bli-passive)
• Boken skrives av læreren. (-s passive)
  Both mean “The book is being written by the teacher.” The choice is largely stylistic; both are correct and common.

They’re the present and past tense of bli:

• Boken blir skrevet av læreren. → present: “The book is (being) written…”
• Boken ble skrevet av læreren. → past: “The book was written…”
  Use ble
  • past participle to talk about completed actions in the past.

The suffix -en on bok makes it definite (“the book”). Because the sentence refers to a specific book known to speaker and listener, you say boken. If you mean “a book is being written…,” you would use the indefinite:

• En bok blir skrevet av læreren.

You have two main options:

1. Stative (resultative) passive with er:
   
   • Boken er skrevet av læreren.
     (“The book is written by the teacher” – implies it’s finished.)
2. Perfect dynamic passive with har blitt:
   
   • Boken har blitt skrevet av læreren.
     (“The book has been written by the teacher” – focuses on the action’s completion.)

Yes, though the neutral order is subject–verb–agent. For emphasis you can front the agent (more common in formal/literary style):

• Av læreren blir boken skrevet.
  This highlights læreren (“It is by the teacher that the book is being written”), but it’s less common in everyday speech.