Suddenly, the sentences are longer. The -te iru form now has four new meanings. Keigo (honorific language) appears, and your brain freezes.

This is the "Intermediate Wall." Most learners crash here because they try to read the grammar book without doing the work.

The textbook shows you plain form. The Renshuu forces you to translate: "I go to Tokyo" (casual to friend) vs. "I will go to Tokyo" (polite to boss). You will fill in 30+ gaps until it hurts. That is good.