The keyboard shortcuts that you can use while watching Prime Video on a web browser, or through the Prime Video apps for Windows and macOS are:

SPACE to toggle play/pause.

F to enter or exit full screen.

(on macOS the shortcut is the FN key and F)

Esc to exit full screen or exit playback.

Left arrow to rewind 10 seconds.

Right arrow to fast-forward 10 seconds.

Up arrow to increase volume.

Down arrow to decrease volume.

M to toggle mute.

C to toggle subtitles/captions on and off, and to switch between available subtitle/caption languages.

A to toggle through available audio tracks, including audio descriptions and Dialogue Boost.