Learn Croatian: Spustili su svjetiljku nad stol. - They lowered the lamp over the table.

Because the verb spustiti (to lower) describes motion toward a destination. When prepositions like nad (over) show where something is moving to, they take the accusative case. If the lamp were already hanging over the table and not moving, you would use the instrumental to show location: Svjetiljka je nad stolom.

Stol is in the accusative case here, so why doesn't its ending change?

Stol is an inanimate masculine noun, meaning it represents an object rather than a living being. For these nouns, the accusative form is identical to the nominative. If it were an animate masculine noun, like a dog (pas), it would take the accusative -a ending (nad psa).

Is spustili a completed action (perfective) or an ongoing one (imperfective)?

It is perfective. Spustiti means they lowered the lamp and the action is finished. If they were in the middle of lowering it, you would use the imperfective verb spuštati: Spuštali su svjetiljku nad stol.

Could I start the sentence with svjetiljku if I wanted to emphasize the lamp?

Yes, you can say Svjetiljku su spustili nad stol. Just remember that the auxiliary verb su is an enclitic, which means it must sit in the second position of the sentence.