If you absolutely cannot afford the $39, there is a legal grey area. Synthesia allows you to use the free version indefinitely, but you must restart the app every 20 minutes.
Unlike subscription apps (like Netflix or Adobe), a Synthesia unlock key is a lifetime license . You pay once, and you keep the full version forever, including all future updates (e.g., version 10.6, 10.7, 11.0). synthesia unlock key code full