Вы заснули? Не сплю я

Why do Russians say “не понял” for “I don’t understand”?

Не понял is past tense, “I didn’t understand,” but Russians use it for right now: “huh?”, “what do you mean?”, “come again?” English says “I don’t understand”; Russian reports the moment of not catching it. A woman says не поняла. Said sharply, не понял! can also mean “excuse me?!” when something is out of line.

The neutral present я не понимаю exists too, and is what you’d say about a language: я не понимаю по-русски. Не понял? is the conversational one, a quick reaction in the moment. In the clip a dozing man answers вы заснули? (“did you fall asleep?”) with не понял, then insists не сплю я.