Hey! Thanks for giving our game a chance! As of now the game is still a prototype so some things aren't super clear, sorry for that.
The E button uses what you have in your hand to interact with the enviroment if needed, or just simply interacts with the enviroment if no item is required to do so.
The F key uses items from your inventory directly "onto" you.
Hope this helps! If you have any other questions let me know!