Learn Swedish: Himlen har mörknat, därför går vi hem. - The sky has grown dark, therefore we are going home.

It is built from the adjective mörk (dark). Adding -na creates an inchoative verb, which simply means a verb describing a change of state, meaning "to become [adjective]". So mörkna means "to get dark". Here it is in the supine form, mörknat, because it is used with har (has).

The word for sky is himmel. Why is the definite form himlen instead of "himmelen"?

When a Swedish noun ends in an unstressed -el, -en, or -er, it usually drops the e when adding a grammatical ending. This makes it easier to pronounce, turning himmel + -en into himlen.

Why does the second clause say går vi instead of vi går?

This is the Swedish V2 (verb-second) rule. The word därför (therefore) acts as the first element in its clause, so the verb går must immediately follow it in the second position, pushing the subject vi to the third position.

Why do we use hem here instead of hemma?