kajero

This is the gift of the band. Kajero is not kaj plus ero, and yet the parse is right there in front of you: kaj is the conjunction "and", the second most common word in the whole language, and ero is an ordinary noun meaning a bit or a particle of something. Two everyday words, sitting in order, spelling a third. And the result would mean roughly "and-bit", which is nonsense in any language.

Say it plainly, because this is the trap the word-building habit sets. Esperanto rewards you for looking for joins, and after a few hundred hours you look for them automatically. Kajero came in whole from French cahier and has nothing to do with either half. The root is kajer-.

The test that catches this class of error is meaning, not spelling. If the parse produces a phrase nobody would ever say, the parse is wrong, however clean the letters look. Compare ekskurso in this same band, where both halves are again real morphemes and the build is again wrong.

In use, a kajero is the school exercise book, the squared or lined thing you write in. A bound blank book you would call a notlibro; a printed one is a libro.