abrupta

One root, however it looks. The Latin behind abrupta really is built from two pieces, ab and rumpere, and English keeps the family visible in rupture and interrupt. None of that survives the crossing. Esperanto has no rupt- root and no ab- prefix, and the verb for breaking is rompi at 651, which is the same Latin word arriving by a different road. So abrupt- is indivisible here: a false seam with a true history behind it, the same shape as inspiri.

It is not just subita. Suddenness is only half of it. Subita at rank 234 is neutral - a sudden noise, a sudden change, no judgement attached. Abrupta adds roughness: the thing happened suddenly and without the softening a person would normally give it. An abrupta respondo is curt. An abrupta fino snaps the thread off. When you mean nothing more than unexpected, subita is the honest word and the commoner one.

The physical sense is the older one. Latin used abruptus for a cliff face, and Esperanto keeps that reading: an abrupta deklivo is a sheer one. Where the slope is merely hard work rather than sheer, kruta at 2,294 is the everyday word, and the two overlap comfortably. The declared adverb abrupte is how you will meet the root most often, because it is usually the manner of an action that is abrupt rather than a thing.