"They are artists."
sanatçı = artist (singular) sanatçılar = artists (plural)
Yet in this sentence "onLAR" is pluralizing the noun following it so you use singular form of artist. O sanatçı = He is an artist Onlar sanatçı = They are artists. Sanatçı geliyor = Artist is coming. Sanatçılar geliyor = Artists are coming.
I think this part is relatively easy but unfortunately it might get a little complicated when an adjective is involved. For instance They are creative artists = Onlar yaratıcı sanatçılar