Learn Swedish: Bartendern måste ner till källaren. - The bartender has to go down to the basement.

In Swedish, you can often leave out verbs of motion (like gå or åka) after a modal verb like måste, ska, or vill, as long as there is a directional word that makes the movement obvious. Here, ner (down) tells us the bartender is moving, so måste gå ner shortens naturally to just måste ner.

Since there is no motion verb, could I use the static form nere instead of ner?

No, you still must use ner. Even though the motion verb is skipped, the sentence is entirely about movement towards a destination. Ner implies motion (going down), while nere implies location (already being down there). The directional adverb is exactly what allows you to drop the verb in the first place.

How do you pluralize an English loanword like bartender in Swedish?

It follows a pattern used for several nouns ending in -er. The plural is bartendrar (indefinite) and bartendrarna (definite). It drops the final 'e' in the root before adding the plural endings, which is similar to how the native word sommar becomes somrar.

Could I say ner i källaren instead of ner till källaren?