Learn Swedish: Vittnet hade redan försvunnit i somras. - The witness had already disappeared last summer.

When talking about a specific season in the past, Swedish adds an -s to the end of the word and uses the preposition i. So, i somras means "last summer", while i sommar usually means "this summer" or "this coming summer". You will see this -s on other past seasons and holidays too, like i våras (last spring) or i julas (last Christmas).

Why is the verb försvunnit used here instead of försvann?

Försvann is the simple past tense ("disappeared"). Here, the sentence uses the past perfect tense ("had disappeared"), which requires the helping verb hade followed by the supine form of the main verb. For the strong verb försvinna (to disappear), that supine form is försvunnit.

Why is the word redan placed between the two verbs?

In a standard Swedish main clause, sentence adverbs like redan (already), inte (not), and ofta (often) are placed right after the first verb. Here, the first verb is the helping verb hade, so redan jumps in before the main verb försvunnit.

Wait, vittnet ends in a -t. Is the basic word for witness vittn?