malplej

The fourth corner of a very small table. Esperanto compares with four invariable words and never changes the adjective itself: pli more, plej most, malpli less, malplej least. Bona, pli bona, la plej bona covers good, better, best; malpli bona, la malplej bona covers the descending half. Four words, no irregular forms, nothing to memorise beyond the pattern. The whole system is set out in comparisons.

Malplej bona and plej malbona are not the same claim. This is the distinction the word exists for, and English cannot make it in one move. La malplej bona is the least good: it grants that everything in the group is good and picks the bottom of the good ones. La plej malbona is the worst: it says the thing is bad and picks the worst of the bad. Where the prefix sits decides which claim you are making, and the same logic runs through the whole system - malpli granda (less big) against pli malgranda (smaller). In casual speech people are loose about it; in careful writing the difference is real and worth using.

How to build the sentence. Malplej normally takes la in front of it when it qualifies an adjective before a noun, exactly as plej does: la malplej kara libro. The group you are choosing from goes in with el: la malplej kara el la tri. With a verb, no article: tio plaĉas al mi malplej. And malplej modifies adverbs as happily as adjectives - li kuras malplej rapide el ĉiuj.

Honestly, it is rare. Speakers reach for la plej mal- far more often than for la malplej, partly because the reversed adjective usually exists and is shorter, and partly because the least-of-the-good claim is a fussier thing to want to say. Recognise it everywhere, and use it when the distinction in the second paragraph is the point you are making.