Structure

The verb structure consists primarily of suffixes:

(i) verb stem

(ii) bound pronominal morphemes, cross-referencing the person and number of core argument(s)

(iii) a TAM suffix

(iv) non-medial verbs that are fully inflected cross-reference the gender of the topical argument.

(iv′) other non-medial verbs and same-subject medial verbs mark nothing else

(iv″) different-subject medial verbs have a form indicating both aspect and switch-reference.

There are some verbs that are exceptions to this structure, such as the Definite Future suffix which requires no gender marking, and some TAM morphemes in medial verbs.

Valency

Siwai has four types of valency structure:^{[1]: 118–9}

(a) plain transitives take A and O

(b) extended transitives take A, O, and E

(c) plain intransitives take S

(d) extended intransitives take S and E

Some verbs are ambitransitive and take either active or middle voice. The voice system of the language is thus a "verbal diathesis" where the configuration of core arguments determine the active or middle voice.

Classes

There are five main verb classes, which are determined by which cross-referencing morphemes they take:^{[1]: 121–6}

(1) S_a verbs

(2) S_a verbs

(3) Irregular verbs ('be, exist', 'go', 'come', 'die', 'cry')

(4) Ambitransitive (active-middle) verbs

(a) 'reflexive action' (S=O=A)

(b) 'spontaneous process/event' (S=O)

(c) 'less agentive activity' (S=A)

(5) Middle-only verbs

(a) 'bodily action'

(b) 'spontaneous process/event'

(c) 'complex activity'