The definite conjugation

Hungarian verbs conjugate differently depending on whether their object is definite or not — a distinction English doesn't make at all. Olvasok egy könyvet ('I'm reading a book') uses one set of endings; Olvasom a könyvet ('I'm reading the book') uses another, purely because a is definite and egy isn't.

The trigger is the object, not the verb: a noun with a/az, a proper name, a third-person pronoun (őt, őket), or even a whole hogy-clause all count as definite and pull in the definite endings — as you already saw in Tudom, hogy igazad van from the last lesson.