For it (actor -um- & theme) to rain.
A substantive with actor and theme roles is unexpressed in the sentence.
Immudan at inudanāna nan nihābal ta way inatdan pinumhod.
(It) rained and so it rained on the tended crops so that they would become robust.
For rain (actor & theme) to rain on something (loc ref -an).
Non-focused actor and theme is a third person singular pronoun, -na.
Ad a''u ya inudananah Benwag.
Last night it rained on Benwag.