Learn Hardcore Norwegian: Frimerket er på brevet.

Frimerket is the definite singular form of the neuter noun frimerke. In Norwegian you don’t use a separate word for “the” in front of the noun; instead you add a suffix. Neuter nouns get -et (common gender get -en). So frimerket literally means the stamp.

Why is brevet also ending in -et?

Same rule: brev is a neuter noun meaning letter, and brevet is its definite form (the letter). The -et suffix marks it as definite singular.

Could I say et frimerke er på brevet instead?

You could say et frimerke er på brevet, but that shifts the meaning to “a stamp is on the letter” (indefinite). Which suggests you’re mentioning one among several possible stamps. In the original sentence the stamp is specific—hence the definite form frimerket.

What exactly does the preposition på mean here? Could I use i instead?

Here på means “on” in the sense of “lying on top of” (frimerket physically sits on the surface of brevet). You wouldn’t use i (“in”) because the stamp isn’t inside the letter; it’s on its outside.

Can I invert the sentence to ? Does that sound natural?