In Italian, I could say Lo sai cosa è successo? ("Do you know what happened?") for introducing a new topic. I would not generally expect an answer, as it is just a way to let the other person know I am changing topic; in the case I see the other person is not paying attention, maybe because he is thinking to something else, or something else is distracting him, I would not continue until he doesn't reply back with Cosa? ("What?") or a similar phrase. If the person to whom I am speaking would be paying attention, and he notices I am not saying anything, he would ask Cosa?
Would "Do you know what happened?" be understood in the same way from English native speakers? What would happen if I say "Do you know what happened?" and I don't say anything else?
I am speaking about a face-to-face communication.
Best Answer
The ordinary phrase in US English is "You know what?" It is spoken with a falling intonation on know and a strong rising intonation on what:
This is employed to announce that you are about to reveal some exciting piece of information, presumably new to your hearers:
It has the prosodic contour of a question, but it doesn't "expect" an answer: it's a rhetorical question. But the speaker may pause to build suspense until they get an answer, which is normally "What?".
The "what" piece may be expanded to a full free relative clause, as in your Italian example, and in that case "what" may be replaced by another more appropriate interrogative:
In these the pitch-stress moves onto the piece after what or who:
There's also a version with the opposite intonation:
This is typically employed to announce new information which is not exciting but unexpected and perhaps unwelcome: