If you are using the KeyDown event, then that's using a code that is "pre translation" and hasn't been changed to actual Unicode values. You can't assume that that will be "lowercase 'a'" because the effects of Caps and Shift keys aren't considered at that point.
The KeyPress event has the translated KeyChar value, the KeyDown and KeyUp events do not.
Use the Keys enum to work out which key it is: Keys Enum (System.Windows.Forms) | Microsoft Docs
] - you will see that there are no lower case keys in there at all!