ŝtopi

One root, and the tempting split is not there. Ŝtop- is indivisible. There is no ŝt- prefix in Esperanto and no -op- suffix that would fit (-op- exists, but it makes collective numbers: duope, in twos). If you have started reading ŝtopi as a build, stop: German stopfen came over whole.

What it actually means, which is narrower than it looks. Ŝtopi is to fill an opening so nothing can pass. The object is the hole, the pipe, the bottle or the ear - not the thing you push in. Li ŝtopis la truon per ŝtofo: the hole is what gets ŝtopita, and the cloth arrives via per. Getting that round the wrong way is the commonest mistake with the verb.

Against its neighbours.

• Plenigi - to fill. About volume, and welcome. A filled glass is plenigita.
• Ŝtopi - to plug. About blocking, and usually a nuisance being dealt with.
• Fermi - to close. About a door or lid, something designed to shut.

The word it grows. Ŝtopilo is the single most useful derivation here: with -il-, the tool suffix, it is the thing that does the plugging, which means a cork, a bath plug and - because somebody had to name it - an electrical plug. Ŝtopiĝi is to become blocked, which is what drains and noses do. Ŝtopita nazo is a blocked nose.

And the domestic sense. In older texts ŝtopi also means to darn: filling the hole in a sock is the same action as filling any other hole, and German stopfen covers both. That is the sense you meet in nineteenth-century Esperanto prose, where somebody is forever mending stockings by a lamp.