Learn Hardcore Norwegian: Jeg kjøper billetter på billettkontoret fordi nettet er nede. - I buy tickets at the ticket office because the internet is down.

kjøper is the present tense of å kjøpe (“to buy”). It can be rendered as “I buy” or “I am buying.” Norwegian uses one form to cover both simple present and continuous/progressive meaning.

How do you form the plural billetter, and what is the singular form?

The singular indefinite is en billett (“a ticket”). To make it plural indefinite, add the ending –er: billetter (“tickets”). If you want the definite plural, add –ene: billettene (“the tickets”).

Why is it på billettkontoret instead of til billettkontoret or i billettkontoret?

Norwegian prepositions with places can differ from English. på is used for many service points and offices: “på posten” (at the post office), “på biblioteket” (at the library).

til billettkontoret would emphasize movement toward it (“to the ticket office”),
i billettkontoret (“in the ticket office”) is grammatically possible but less idiomatic when talking about buying tickets.

What is billettkontoret, and how is its definite form constructed?

billettkontoret is a compound of billett (ticket) + kontor (office). The base (indefinite) is billettkontor. To make it definite singular, you add –et at the end: billettkontoret (“the ticket office”).

What role does fordi play, and how does it affect word order?

fordi means “because” and is a subordinating conjunction. In subordinate clauses after fordi, Norwegian uses Subject–Verb–Object (SVO) order rather than the main-clause Verb–Second (V2) rule. For example:
fordi nettet er nede
Subject (nettet) + Verb (er) + Complement (nede)

Why is it nettet, and can it mean the Internet?

et nett = “a net” or “a network”; adding –et makes it definite: nettet = “the net/network.” In everyday Norwegian, nettet commonly refers to the Internet. So nettet er nede means “the Internet is down.”

What is nede, and why not ned?

nede is an adverb meaning “down,” “down there,” or “out of service.” It doesn’t change form. ned is also an adverb but is used for movement toward a lower position (“gå ned trappen” = “go down the stairs”). Here, nede describes a state (“is down”).

Do I need a comma before fordi in this sentence?

Norwegian punctuation generally omits the comma before fordi when the subordinate clause follows directly.
Jeg kjøper billetter på billettkontoret fordi nettet er nede.
Adding a comma is optional and often done only for clarity or a deliberate pause.