738. The several stems of the verb come from a form called the _Theme_.
In primitives, the theme is a root; in denominatives, the theme is a
noun stem.
Thus, #reg-# in #reg-ō# is a root; while #vesti-# in #vesti-ō#, _dress_,
is a noun stem. The noun stem is sometimes modified in form. Oftentimes
the noun stem is only presumed: as, #audi-# in #audi-ō#.
739. Some verbs have a denominative theme in the present system, and a
primitive theme in the perfect system, others have the reverse.
740. Most verbs with an infinitive of more than two syllables in #-āre#,
#-ēre#, or #-īre#, or, if deponent, in #-ārī#, #-ērī#, or #-īrī#, are
denominative; most other verbs are primitive.
Thus, #laudāre#, #monēre#, #audīre#; #mīrārī#, #verērī#, #partīrī#, are
denominative; while #esse#, #dare#, (#dē#)#lēre#, #regere#, #querī#, are
primitive. A few verbs, however, which have the appearance of
denominatives, are thought to be primitive in their origin.