Textbook vs Real
Textbook이제 시작입니다i-je si-jak-im-ni-da
Real이제 시작이야i-je si-jak-i-ya
Why Koreans say it this way
Dong-eun's revenge is warming up. 이야 (casual "it is") vs 입니다 (formal). The casual ending makes it personal and threatening — it's not an announcement, it's a promise directed at someone.
From: 더 글로리 EP8