Esperanto · Chapter 15 · Lesson 14 · Grammar primer
The -um wildcard, and word-formation beyond the rules
Plenumi, brakumi, komunumo: -um is the official 'no rule applies' suffix, which means -um words are vocabulary. Plus the root-class logic behind why marteli works and krajoni does not.
The short version
-um- is the suffix that promises nothing
Every other affix computes: -il- is the tool, -ej- the place, -ig- to cause. -Um- is the official exception, marking a word as having something to do with the root and refusing to say what. Kolumo is a collar, brakumi is to hug, aerumi is to air a room, plenumi is to fulfil, and they share nothing but the wildcard. Keep malvarmigi, which computes to cooling the soup down, next to malvarmumi, which is pure convention and means catching a cold. Learn them; do not coin new ones.
Mi malvarmumis.
Marteli is a word; krajoni never became one
Swapping endings always yields a grammatical word, but what it means depends on the root's class, which nothing on the surface shows. Bros- is a thing-root, so broso is the brush and brosi derives from it; komb- is an action-root, so kombi is the combing and the tool has to be built as kombilo. Two near-identical objects, opposite directions. A thing-root verbs only where one action is bound to the thing: marteli yes, krajoni no, because you simply skribas per krajono.
broso donas brosi, sed kombi donas kombilon
That's the gist. The full article goes deeper - more examples, the edge cases, the why - and the practice below scores like a vocabulary session and earns the same XP.
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