Learn Danish: Da jeg ankom, havde han allerede spist. - When I arrived, he had already eaten.

In Danish, you form the past perfect using either havde (had) or var (was) plus the past participle. Most verbs, like spise (to eat), use havde. You generally only use var with verbs that describe movement from one place to another or a change of state.

Why does the second part of the sentence say havde han instead of han havde?

This happens because of the V2 (verb-second) rule. The entire first clause (Da jeg ankom) counts as the first position in the sentence. Therefore, the main verb (havde) must take the second position, forcing the subject (han) to move after it.

Where exactly do we put adverbs like allerede in a sentence with a past perfect verb?

When a verb has two parts (like havde and spist), short adverbs like allerede (already), ikke (not), or ofte (often) go right between the subject (han) and the second verb (spist) in an inverted main clause like this one.

Could I use Når instead of Da for the word 'when' in this sentence?

No, you must use Da. Danish uses da for a single, specific event that happened once in the past. Når is used for the present and future, or for repeated events in the past where it means 'whenever'.