The difference between знать, узнать, узнавать
Знать means "to know", when you already know something or somebody.
Я уже знаю это правило. I already know this rule.
Узнать means "to find out" or "to recognize".
Я узнал тебя! I have regognized you!
Узнавать is the imperfective form of "узнать", its main meaning is "to recognize".
Ты меня узнаёшь? Are you recognizing me?
So, we can see two pairs of imperfective and perfective verbs: