-uu

From earlier *-au < *-ao < *-ado, from Proto-Finnic *-o (whence also Finnish -o) plus the inflectional stem of verbs ending in *-at'ak (stem *-ada-). Cognates include Karelian -avo.

-uu (front vowel harmony variant -yy, linguistic notation -UU)

1. Forms nouns from verbs.

   ‎evätä (“to deny”) + ‎-uu → ‎epuu (“refusal”)

   ‎hakata (“to beat, chop, cut”) + ‎-uu → ‎hakkuu (“chopping, logging”)

   ‎kerätä (“to collect”) + ‎-uu → ‎keruu (“collecting”)

• This suffix is used for some verbs whose citation form ends in -ata/-ätä.
• The front-harmonic variant -yy would, in theory, be used when the first vowel of the word is one of the harmonic front vowels -y-, -ä-, -ö- (stems containing only neutral front vowels -e-, -i- take the back-harmonic variant). Standard Finnish has no words using -yy, however, but they can be found in dialects.
• The verbal stem is in the strong grade if applicable.

• -o
• -u