This is the 924th most frequent Swedish word.
regn
Rain.
Here, 'regnet' refers to the rainfall itself as a natural phenomenon, used as a noun in the definite form.
Regnet öste ner hela kvällen.
The rain poured down the entire evening.
In this context, 'regn' is used to describe rain in an indefinite and general sense.
Under semestern träffade vi på regn varje dag.
During the vacation, we encountered rain every day.
Here, 'regna' is the verb form derived from 'regn', indicating the action of raining.