rojo

Rojo is a small running watercourse. The Reta Vortaro defines it precisely and, unusually, comparatively: natura akvofluo, pli malgranda ol rivereto, ordinare ne surnavigebla - a natural watercourse, smaller than a rivereto, ordinarily not navigable. So the sizes stack: rivero a river, rivereto a stream, rojo smaller still. In English that is a rill, or a brook if the water is a bit braver.

Worth knowing before you use it: the same dictionary entry carries a remark recommending rivereto over rojo in ordinary writing. That is not a rule, and rojo is perfectly good Esperanto with a century of use behind it, but it tells you which of the two a reader will find unremarkable. Rivereto is transparent, being rivero plus the diminutive -et-, and Esperanto tends to prefer the piece it can take apart.

The near-pairs. Three short words differ by a single sound and mean nothing like each other: rojo a rill, roso dew, and rozo a rose. Add ruĝa, red, and you have a small pronunciation drill worth doing out loud. Esperanto's j is the y of yes, so rojo is two syllables, ROY-o, and never rhymes with rozo.