sata

You met the derived word first, and that is the interesting part. Malsata, hungry, sits at rank 1057 in this course. Sata, its base, sits here at 2186 - over a thousand places later. That is the wrong way round if you think of mal- as a suffix that arrives after the root is known. In practice Esperanto speakers say "hungry" far more often than "not hungry", so the negated form is the common one and the positive is the specialist. The reversal is real, it is frequent enough to be a pattern, and it is worth decoding plainly rather than hoping it feels natural.

How to hold it. Do not think of sata as "un-hungry". Think of malsata as "un-full". Sata is the primary state - you have eaten, you want no more - and mal- flips it to the state of wanting food. The same reversal shows up elsewhere in the language: malsana (ill) is far commoner than sana (healthy) in day-to-day talk, and malbona beats bona in complaints. The prefix is doing exactly what it always does; it is only our expectation about which word comes first that is off.

Not the same as plena. Plena is full in the container sense - a full glass, a full room, a complete list. Sata is full in the eater's sense and applies to living things. La telero estas plena is a plate with food on it; mi estas sata is a person who has had enough of it. Getting these crossed produces the classic learner's sentence in which the speaker announces they are a full glass.

Beyond food. Sata je io is fed up with something - the metaphor runs the same way English does with "I have had enough of this". Sateco is the state of being full, and satigi is to feed someone until they are.