kurba

One root, and no seam to look for. Kurba came in whole from Latin curvus through French courbe, and there is nothing inside it: kurb- is the root, -a is the adjective ending, and that is the entire analysis. Say it "KOOR-bah", with a rolled r if you have one and a clear u, not the English cur.

The pair to fix it against is rekta. Rekta is straight, and it is the more frequent of the two by a wide margin, at 518. Esperanto is unusually tidy here because both adjectives keep their literal and figurative senses in step with English: a rekta vojo is a straight road and a rekta respondo is a straight answer, while a kurba vojo winds and kurbaj vojoj in the moral sense are crooked ways. Malrekta also exists and is not quite a synonym of kurba: malrekta is "not straight", which covers bent, askew and off-line, whereas kurba specifically follows a curve. A crooked picture on a wall is malrekta; a banana is kurba.

The shape words, sorted. These four get muddled constantly, so it is worth laying them side by side. Kurba is about a line that bends. Glata is about a surface with nothing rough on it, smooth to the touch. Plata is about a surface with no relief, flat. Ronda is about a shape closing on itself, round. A road can be kurba and glata at once, a plata field can have a kurba river across it, and none of the four implies any of the others.

Building on it. Kurbo is a curve as a noun, and it is the word mathematics and statistics use: la kurbo de la temperaturo. Kurbigi is to bend something, kurbiĝi is to bend or to warp of its own accord, and that pair is worth a moment because it is the igi and iĝi distinction in its clearest form. Kurbeco is curvature. And the derived adverb kurbe means along a curved path, which is more useful in describing movement than the adjective is.