Learn Danish: Da hæren ikke var her, angreb fjenden byen. - Since the army was not here, the enemy attacked the city.

While both can translate to "because," da is usually used like the English word "since." You use da when the reason is already known, established, or acts as background information setting the scene. Fordi is typically used when you are giving a new, direct reason for something.

Why is ikke placed before the verb var? Usually it comes after the verb.

The word da starts a subordinate clause (a sentence part that cannot stand alone). In Danish, a major rule for subordinate clauses is that sentence adverbs like ikke must come before the verb. If this were a simple main clause, it would be Hæren var ikke her.

Why does the second half of the sentence say angreb fjenden instead of fjenden angreb?

This is because of the Danish V2 (verb-second) rule. The entire first clause (Da hæren ikke var her) takes up position number one in the overall sentence. To keep the main verb in the second position, the verb angreb must come immediately after the comma, before its subject fjenden.

Angreb looks like a noun. How do we know it is a verb here?

Angreb is the past tense of the strong verb at angribe (to attack). You can tell it is functioning as a verb here because it sits directly in the verb slot required by the V2 rule right after the comma. It just happens to be spelled exactly like the noun et angreb (an attack).