Conjugation in Real Life
Brachylog, 90 bytes

;Ṡ{hhH↰₂gB&amp;hb~c[A,B,C]&amp;tT;C↔↰U∧" under ",H,T;A↔↰,U|tT&amp;hh,TgJ&amp;hb;T↰Q∧J,Q|h}
~↰₃|↰₃
∧"un";?c


I made this while being under the impression that this could be recursive and multiple unders could be stacked. Probably not optimized. Also as this is bracylog, plumbing takes quite some bytes.

Try it online!