enfermi

A genuine decode, sitting one rank away from a false one. This band is mostly a lesson in not splitting borrowed roots, so it is worth stopping on the case where splitting is right. En is in. Fermi is to close. Enfermi is to close somebody or something in. Both halves are real Esperanto words, and they combine to exactly the meaning the compound has, which is the test.

Compare it with kapitalo at rank 2,608 or preventi at 2,607, where the visible pieces are Latin fossils. Same neighbourhood, opposite verdict. The pieces here are alive.

What the en- adds. Fermi acts on the opening: you close a door, a box, a book. Enfermi acts on the contents: you shut in the dog, the prisoner, the smell. Notice that the object changes entirely. Fermi la ĉambron is to close the room's door; enfermi la hundon en la ĉambro is to leave the dog on the wrong side of it.

Where the preposition goes. The en inside the verb has already been used up, so the location needs its own: enfermi ion en io. And that inner en takes the nominative, not the accusative of direction, because the shutting already happened and the dog is now simply there: en la ĉambro, not en la ĉambron. This trips up almost everybody once.

Related builds worth having. Enfermejo is a place of confinement, malliberejo is the ordinary word for a prison, and enfermiĝi is to shut oneself away, with -iĝ- turning the action back on the subject. The opposite move is liberigi, to set free.