"After your Amazon Prime or Prime Video membership or promotional trial expires or is cancelled, you cannot watch any video available through your Prime or Prime Video subscription unless you've rented or purchased that title."
Your thread title includes the word "rent" but your OP says "free video." Which type are you asking about? Per the terms of service link I posted, if you've rented a video you can watch it after you've cancelled for the time period before it expires (30 days to start it, and then 48 hours after you've started, I believe). If it's a "free" download, you're not able to watch it once you cancel.
You need to read the terms of service to understand the different scenarios.