From Proto-Cariban *-rɨ. The development into an imperfective marker is relatively late, and a similar evolution can be observed in the Kari'na cognate -ry.
-dü
This suffix does not cause syllable reduction.
Verbs nominalized with this suffix take person markers of series II; however, when the verb is intransitive and its argument is explicitly expressed as a noun before the verb, no person markers are used. Nonetheless, even in this latter case, if the argument is marked as an oblique argument of another verb in the sentence, person markings must be expressed.
When a verb takes this suffix to form the imperfective aspect, it can function syntactically as a main verb, but morphologically it behaves like a nominalized form: it takes the intransitive marker w-, series II person markers that index either an intransitive argument or a transitive patient, and the nominal plural suffix -komo and negation -jünü instead of the verbal plurals -to and -tö. Furthermore, the transitive agent, when expressed, is marked by the postposition -uwö instead of unmarked (as it would be with series I markers).
Perhaps ultimately of the same origin as etymology 1 above; a similar construction can be found in Pemon -yi, also homophonic with one of the allomorphs of the Pemon possessive marker.
-dü
This morpheme is properly a clitic rather than a suffix. It causes syllable reduction when it follows the recent past imperfective suffix -anö, in which case this morpheme takes the form -nü to make the combined form -annü. Another allomorph, -chü, is used when the preceding stem ends with i, typically as either the past tense or the unreal suffix -i. In all other cases the form of this clitic is -dü.
The finite verb in the nominalized construction must take series I person markers.