Final answer:
An astronaut falling into a black hole would experience time dilation as they near the event horizon, would be unable to communicate with the outside world upon crossing it, and would eventually be stretched and compressed by tidal forces in a process known as spaghettification.
Step-by-step explanation:
When an astronaut falls into a black hole, they would experience extreme gravitational forces. As they approach the event horizon, time would begin to slow down relative to outside observers due to the intense gravity of the black hole, which warps spacetime. If they could send a signal at regular intervals, these signals would become increasingly redshifted and eventually stop as observed from the outside, indicating they've crossed the event horizon.
Inside the event horizon, communication with the outside universe becomes impossible, and the astronaut would face a phenomenon known as spaghettification.
This happens as the gravity at their feet would be significantly stronger than at their head, stretching the body out. The extreme stretching and tearing would depend on the black hole's size, with more massive black holes causing spaghettification only after crossing the event horizon, while smaller ones would do so even before crossing it.
If the astronaut is associated with a spacecraft that gets trapped inside the event horizon, any attempt to escape, including sending radio messages, would be futile as the curvature of spacetime inside the black hole ensures that all paths lead to its center, and thus all signals are also trapped, never reaching outside observers.