It's up to the player to make modifier keybinds, those are not default standard keys. Shift for sprinting and Ctrl for crouch are standard keys in fps games. We should be thinking standard defaults and then working around that. It's really simple if you leave shift for sprint only, ctrl for crouch only. The rest of the keyboard is then wide open. You can have ALT dedicated to alternative use, being the Use key with ALT and it makes sense because it's the same key but with ALT. From there I would make sure nothing else uses SHIFT, ALT or CTRL for defaults. The player can change keybinds if they need to but everything needs to be standardized and simple to play with.