The nominative case expresses the subject or predicate (verb) of the sentence. Nominative is the same as the base form of a word, that’s why it doesn’t have any ending in singular. The plural ending is -t.
Mun koira on söpö. <> Mun koirat ovat söpöjä.
My dog is cute. <> My dogs are cute.
The plural ending -t is added to the stem of the word. Depending on the word type, the stem may be different from the base form. Note also the KPT changes.
pieni tyttö <> pienet tytöt
kaunis nainen <> kauniit naiset
uusi puhelin <> uudet puhelimet