memkomprenebla

Take it apart slowly, because every piece is one you already have.Mem is self. Kompren- is understand. -ebl- is the suffix meaning able to be done, the one in videbla (visible), manĝebla (edible) and nekredebla (unbelievable). -a is the adjective ending. Assemble: able to be understood by itself, which is self-evident, obvious, a thing that needs no explaining. Four morphemes, no irregularity, and a fourteen-letter word you can read cold the first time you meet it.

Is it the longest build in the course? Not quite, and it is worth being honest about it. By letters, yes: fourteen beats malmultekosta at thirteen. By pieces, no. Malmultekosta (cheap) is mal- + mult- + -e- + kost- + -a, five, and it carries an internal adverb into the bargain, which is the harder trick. Mallongigo (an abbreviation) is four, the same as this one. So memkomprenebla wins on length and comes second on structure, and if you can decode both you can decode anything the language is likely to throw at you.

Where you will actually meet it is the adverb. Memkompreneble at the head of a sentence is Esperanto's of course - warmer and less clipped than kompreneble, which does the same job and is far commoner. Between the neighbours: klara is clear because it is well put, evidenta is evident because the proof is in front of you, and memkomprenebla is obvious because it could not sensibly have been otherwise. Note the small trap in the negative: nememkomprenebla is legal and horrible, and every good writer says ne memkomprenebla instead.