Den nominelle renten er lav, men den effektive renten er høy.

This is the normal Bokmål pattern called double definiteness.

• rente = interest rate
• renten = the interest rate
• den nominelle renten = the nominal interest rate

When a noun is definite and has an adjective before it, Norwegian usually uses:

• den/det/de before the adjective
• and the noun in its definite form

So den nominelle renten is the expected structure. Without an adjective, you would normally just say renten.

Because adjectives before a definite noun normally take the -e form in Bokmål.

Compare:

• en nominell rente = a nominal interest rate

• den nominelle renten = the nominal interest rate

• en effektiv rente = an effective interest rate
• den effektive renten = the effective interest rate

So the -e ending is there because the noun phrase is definite.

Because lav and høy come after er, so they are predicative adjectives, not adjectives directly before the noun.

Predicative adjectives do not take the definite -e just because the noun is definite. Instead, they agree mainly with gender and number.

Here the subject is singular common-gender renten, so the correct forms are:

• renten er lav
• renten er høy

Compare:

• nivået er lavt = the level is low

• rentene er lave = the interest rates are low

• nivået er høyt = the level is high
• rentene er høye = the interest rates are high

In Bokmål, many nouns can be treated either as feminine or as common gender. Rente is one of them.

So you may see both:

• en rente – renten
• ei rente – renta

In standard written Bokmål, especially in formal or financial language, en rente / renten is very common. That is why this sentence uses renten.

Yes. That is natural Norwegian.

Because renten is already understood from context, you can shorten the second part:

• Den nominelle renten er lav, men den effektive renten er høy.
• Den nominelle renten er lav, men den effektive er høy.

Both are correct. The full version is a little more explicit; the shorter version avoids repetition.

Men means but. It joins two main clauses:

• Den nominelle renten er lav
• den effektive renten er høy

After men, the second clause has normal main-clause order here:

• subject: den effektive renten
• verb: er
• adjective: høy

So men itself does not force a special word order in this sentence.

Here, den is part of the definite noun phrase, so it corresponds to English the in a structure like the nominal interest rate.

In other contexts, den can also mean that or it, but not here.

So in this sentence:

• den nominelle renten = the nominal interest rate
• den effektive renten = the effective interest rate

It is best not to read den here as a separate pronoun meaning it.

Use the indefinite article en and the basic adjective form:

• en nominell rente
• en effektiv rente

So the contrast is:

• en nominell rente = a nominal interest rate

• den nominelle renten = the nominal interest rate

• en effektiv rente = an effective interest rate
• den effektive renten = the effective interest rate