Como dizer "Não entendi; Eu não entendi" em inglês

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Cf. Como dizer "eu entendo e eu entendi" em inglês

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I didn’t understand you.
When you don’t understand what someone just said because they mumbled their words, they used unfamiliar vocabulary, or there was a bad connection and the words were garbled.

Example:
Bob: So, we’re going to mumble, mumble the customer’s requirement and then give it to mumble.
Mary: I didn’t understand what you just said.
(Bob will repeat himself more clearly)

I didn’t understand it.
When you understood the words but not the content.
Doug: I went to the lecture on astrophysics but I didn’t understand it.

I don’t understand
When you don’t see the rationale for something or when you are confused.