sub

Sub is where the accusative earns its keep. Sub la tablo is a position, sub la tablon is a movement to that position, and the only difference is the -n. A handful of prepositions work this way (sub, sur, en, antaŭ, inter); the rest, like tra and sen, never need it because their meaning already settles the question. As a prefix the root gives subiri (to go down, to set) and subtera (underground).