Bonjour
Isabelle et David comprennent bien le français.
If you learn the conjugation of the verb 'prendre', then you'll know all those ending by 'prendre' as 'comprendre' 'apprendre' reprendre' entreprendre' etc....
They drop the 'd' at the 3 plural person and double the 'n' at the 3rd pers. plural
prendre
je prends
tu prends
il/elle/on prend
nous prenons
vous prenez
il/elle prennent ===> Isabelle et David = ils comprennent