A verb is a necessary part of any sentence, except when making copular sentences. Verbs can be categorised on the basis of how many elements compose them.
The class of stem verbs is semi-closed — there are only about 100 pure verbs in the entire language. Stem verbs consist of a single element.
In order to extend or specify meaning, verbs can be compounded much as a noun can. Compound verbs are, however, much more common than compound nouns, to make up for the relative lack of stem verbs.