Learn Hardcore Norwegian: I hverdagen betaler jeg fakturaene med en gang, ellers glemmer jeg dem. - In everyday life I pay the invoices right away, otherwise I forget them.

Norwegian often begins a sentence with a time/place phrase to set the context. When something other than the subject comes first, Norwegian still follows the V2 rule (the finite verb must be the second “slot”), so you get:

I hverdagen (slot 1)
betaler (slot 2, the verb)
jeg (the subject comes after the verb)

So this start is mainly about emphasis and style: In everyday life / Normally, I pay...

What exactly does i hverdagen mean, and is it idiomatic?

Yes, it’s idiomatic. I hverdagen literally means in the everyday (life) and is commonly used to mean in day-to-day life / in everyday situations / normally.
A close alternative is til hverdags, which also means in everyday life and often sounds a bit more like on an ordinary day / typically.

Why is hverdagen in the definite form (-en)?

Norwegian often uses the definite form in fixed expressions where English would use a general/abstract noun. Hverdagen (the everyday life) works like a general concept. You’ll see this pattern in other expressions too, where the definite form sounds more natural and idiomatic than the indefinite.

Why is it fakturaene (definite plural), and how is that formed?

Fakturaene means the invoices/bills (specific ones the speaker has). Formation:

singular: en faktura
plural indefinite: fakturaer
plural definite: fakturaene (often written as fakturaene, sometimes fakturaene depending on the word; for faktura it’s fakturaer → fakturaene)

Using the definite plural matches the idea of a known set: the bills you regularly get.

Could I replace fakturaene with regningene? What’s the difference?

Yes, very often.

en regning / regningene = a bill / the bills (general everyday word)
en faktura / fakturaene = an invoice / the invoices (often a bit more formal/business-like)

If you’re talking about household bills, regningene is extremely common; fakturaene is also fine, just slightly more “invoice-like.”

What does med en gang mean here, and does it literally mean “with one time”?

It means immediately / right away / straight away. It’s an idiom; don’t translate it word-for-word.
Common near-synonyms:

med det samme = immediately
straks = immediately (a bit more formal)

Also note the contrast:

med en gang = immediately
på en gang = all at once (different meaning)

Why is there a comma before ellers?

The comma separates two independent clauses: 1) I hverdagen betaler jeg fakturaene med en gang
2) ellers glemmer jeg dem

This is like English: ..., otherwise .... It’s a natural place for a comma because ellers introduces the consequence/alternative.

How does ellers work grammatically—does it mean otherwise or if not?

It can cover both ideas depending on context. Here it means: otherwise / if I don’t (do that).
So the logic is: I pay them immediately; if not, I forget them.
Placed at the start of the second clause, ellers also triggers V2 word order (see next question).

Why is it ellers glemmer jeg dem and not ellers jeg glemmer dem?

Because of the V2 rule again. When an adverb like ellers is in the first slot of a main clause, the finite verb must come next:

ellers (slot 1)
glemmer (slot 2)
jeg (subject after the verb)

So ellers jeg glemmer would break standard Norwegian word order.

Why do we repeat jeg in the second part? Can we omit it?

In Norwegian you generally need an explicit subject in each main clause. Since ellers glemmer jeg dem is its own clause, it needs jeg.
You can’t normally drop it the way some languages do. The repetition is completely natural.

What does dem refer to, and why is it dem instead of de?

Dem refers to fakturaene (the bills/invoices).
Norwegian distinguishes subject vs object pronouns:

de = they (subject)
dem = them (object)

Here the bills are the direct object of glemmer (forget), so you need dem.

Is the present tense (betaler, glemmer) used to talk about a habit?

Yes. Norwegian uses the present tense for habitual actions and general truths, just like English:

I hverdagen betaler jeg... = In everyday life, I pay... (as a routine)
ellers glemmer jeg... = otherwise I forget... (what tends to happen)

Any pronunciation tips for tricky words in this sentence?

Common sticking points:

hverdagen: the hv is pronounced like v (so it starts with a v sound).
fakturaene: stress is typically on the last part of faktura (faKTuRA in many dialects), and the ending -ene is a separate syllable.
glemmer: double mm signals a short vowel before it (a “tighter” e sound).