Learn Danish: Tyven opdagede at hun havde låst døren. - The thief discovered that she had locked the door.

The sentence talks about two things that happened in the past. We use the past perfect (førdatid) to show which action happened first. She locked the door before the thief discovered it, so the earlier action becomes havde låst.

Why does this sentence use havde instead of var for the past perfect?

In Danish, most verbs use have (here in the past tense as havde) to form the past perfect. You only use være (var) for verbs that describe a change of state or movement from A to B (like dying, going, or arriving). Locking a door is a standard action, so it takes havde.

What would happen to the word order if we wanted to say she had NOT locked the door?

The word at introduces a subordinate clause (a dependent part of the sentence). In Danish subordinate clauses, adverbs like ikke must be placed before the verbs. So it would become: Tyven opdagede at hun ikke havde låst døren.

How do you build the past participle låst?

Låst is the past participle of the verb at låse (to lock). For many regular verbs in Danish, you form the past participle by removing the final -e from the infinitive and adding a -t.