Hvis renten stiger, blir det vanskeligere å nedbetale både lån og gjeld.

Renten is the definite form (“the interest rate”), referring to a specific interest rate that’s already understood in context (often the general/official rate or your rate).
En rente would mean “an interest rate” in a more general, non-specific sense.

Norwegian normally uses a comma between a subordinate clause and the main clause when the subordinate clause comes first:
Hvis renten stiger, (subordinate clause) blir det ... (main clause).

Yes. Hvis introduces a conditional subordinate clause. The word order inside that clause is straightforward: subject + verb:
renten (subject) + stiger (verb).

Because Norwegian has V2 word order in main clauses: the finite verb is in the second position.
When the sentence starts with the Hvis-clause, that whole clause counts as the first element, so the main clause must start with the verb:
Hvis …, blir det …
(Compare: Det blir vanskeligere … if the main clause stands alone.)

Here det is a dummy/expletive subject, like English “it” in “it becomes harder to…”. It doesn’t refer to a specific noun; it just fills the subject slot:
Det blir vanskeligere å ... = “It becomes harder to ...”

Vanskeligere is the comparative form: “more difficult / harder.”
It’s formed with -ere for many adjectives:
vanskelig → vanskeligere.

Å is the standard infinitive marker, like English “to”:
å nedbetale = “to pay down / to repay.”

Nedbetale means “to pay down” (typically gradually, in installments) or “to repay.”
A common alternative phrasing is betale ned (same idea, just written as a verb + particle).

Både … og … means “both … and …” and links two items in a balanced way:
både lån og gjeld = “both loans and debt.”
You typically don’t put a comma between the two items.

Yes, though they overlap in everyday usage:

• lån = a loan (a specific borrowed amount/product from a lender)
• gjeld = debt (the total amount you owe, or debt in general)
  So the phrase covers both the specific loans and the overall debt burden.

Norwegian often uses present tense for general conditions and predictable results, similar to English:
“If the rate rises, it becomes harder …”
You don’t need a future marker unless you want to emphasize the future.