redukti

Redukti is transitive and it stays transitive. You reduce a thing: redukti la koston, redukti la nombron, redukti la rapidon. If the thing shrinks by itself, Esperanto makes you say so with the reflexive suffix, and reduktiĝi is the word. English blurs the two ("the numbers reduced", "we reduced the numbers"); Esperanto does not let you, and that is the usual pattern rather than a quirk of this verb. Transitivity has the full list of pairs that catch English speakers out.

Do not reach for it where a plain comparative will do. Malpli is the everyday word for less, and malpliigi is a perfectly good to make less. Redukti carries a faint air of the report and the budget line: costs, quantities, percentages, sentences, fractions. Learners who have met the word in a European language tend to overuse it in exactly the register where a native Esperanto speaker would say malgrandigi.

It has technical lives too, which is worth knowing before you meet them. In chemistry redukti is the opposite of to oxidise; in maths you redukti a fraction; in cooking a sauce reduktiĝas on the heat. All of these come straight out of the Latin, and none of them needs a new word.