The standard finger position is the usual
ASDF JKL; keys.
Regarding the thumbs, the left thumb should rest over the
Backspace key and the right
thumb should rest over the
Space key. Both thumbs share the central
Dual-functions keys are keys that combine a TAP key and a MOD.
Simply speaking: when a dual-function key is tapped, one get the tap effect of the key. When it's holded or combined with another key, it will act as a modifier.
Layers are extra «layouts» containing different keys that can be accessed temporarly when pressing the corresponding layer key.
That's like another
Shift that provides access to different characters.
To learn the layers quickly print the guide.