Learn Danish: Vi hører ulven jage om natten. - We hear the wolf hunt at night.

Because hører (hear) is a verb of perception. Just like in English (we hear the wolf hunt, not to hunt), Danish verbs of perception take a direct object (ulven) followed by a bare infinitive. You drop the at and just use jage.

Could I say Vi hører ulven jager using the present tense?

No, you cannot mix the direct object construction with a conjugated verb like jager. If you want to use the present tense, you have to create a full clause using at (that): Vi hører, at ulven jager om natten (We hear that the wolf hunts at night).

Why does the time expression use om and the definite noun natten?

In Danish, using the preposition om with the definite form of a time period (like natten or morgenen) indicates a recurring action. Om natten means at night or during the night as a general, repeated routine.

Can ulven refer to wolves in general here, even though it is singular?

Yes. Just like in English, Danish often uses the definite singular (ulven - the wolf) to represent the entire species as a general concept, rather than one specific animal.