If you like your spiffy new mechanical keyboard but want to use it on a Mac, you need to map the Command key and Option key around because on a Mac, it goes Control, Alt or Option and then Command whereas on a Windows keyboard it is Control, Windows-Key and then Alt.
This is really confusing for muscle memory. The solution is to remap the Modifier keys, you go to System Preferences/keyboard/modifier keys and then select the USB Keyboard and then for Option Key map to Command Key and vice versa. Done!
Or select the magic keyboard name. This is nice because it is specific just to your specific magic keyboard, so if you put in a regular Mac keyboard it still works.