Bitirmek is transitive. So we need to use an object. Bitmek is intransitive. So it never takes object (accusative). Ben işimi bitirdim. We have an object, thats why we used bitirmek. İş bitti. There is no object. Its intransitive. So it must be bitmek. -r is a suffix that we turn intransitive verbs to transitive ones. There are not many, but common verbs: Doğmak: to be born Doğurmak: to give a birth, to bear Bitmek: to end, to come to end Bitirmek: to finish sth Yatmak: to lie (on the sofa, bed), to go to bed Yatırmak: to put someone to bed, to bed Uçmak: to fly Uçurmak: to fly sth (a kite etc.), to let/make sth/sb fly Btw its better not to use possesive pronouns if you use the subject in the sentence: Ben işimi bitirdim. is better instead of ben benim işimi bitirdim. (İts not grammatically wrong, but doesnt sound nice. Because with the word we already give the person işiMi) and you have to use an -i after işim to give the accusative case: ben işimİ bitirdim.

BEN İŞİMİ BİTİRDİM also second.