Learn Hardcore Norwegian: Luften blir renere etter regnet. - The air becomes cleaner after the rain.

Blir is the present form of å bli and here it means “becomes.” You use bli when describing a change of state. If you said luften er renere etter regnet, it would mean “the air is cleaner after the rain” (a static description). By using blir renere, you emphasize that the air becomes cleaner as a result of the rain.

Why is luften in the definite form? Aren’t general statements usually indefinite?

In Norwegian, when you speak about something in a general or natural context—like the air—you normally use the definite form. Luften (“the air”) refers to the atmosphere in general, so you add the suffix -en to luft. If you said luft blir renere etter regnet, it would sound odd or poetic in everyday speech.

How is the comparative renere formed from the adjective ren?

For most monosyllabic adjectives in Norwegian, you form the comparative by adding -ere. So ren (“clean”) becomes renere (“cleaner”). Some adjectives are irregular (for example stor → større, god → bedre), but ren follows the regular pattern.

Could I say mer ren instead of renere?

Yes, grammatically mer ren (“more clean”) is possible (that’s the analytic comparative), but Norwegian prefers the synthetic form renere for short adjectives. Mer ren sounds more formal or marked, whereas renere is the natural choice.

Why do we say etter regnet and not etter det regnet or etter å regne?

Etter is a preposition that takes a noun phrase.
To say “after the rain,” you add the definite suffix -et to regn, giving regnet.
Using det (a separate article) would be redundant, because Norwegian expresses definiteness on the noun itself.
Etter å regne would literally mean “after to rain,” which isn’t grammatical in Norwegian.

Could we use a full clause like etter at det har regnet instead of etter regnet?

Yes. You can say:
Etter at det har regnet blir luften renere.
Here etter at introduces a subordinate clause (det har regnet = “it has rained”). The nuance is nearly the same, but etter regnet (a simple prepositional phrase) is more concise and very common in everyday speech.

Can we move etter regnet to the beginning of the sentence?

Absolutely. Norwegian is a V2 (verb-second) language. If you start with the adverbial, the finite verb stays in second position:
Etter regnet blir luften renere.
Here blir is still the second element, followed by the subject luften.