#19: The first thing you need to be able to do is count points. Pawn=1, Knight=3, Bishop=3, Rook=5, Queen=9. Blunder checking is about ensuring that you don't go down a path where you inevitably concede material which you could've avoided.
#21: Memorising moves won't help you with blunder checking. You need to be able to calculate forwards from the current position.
In other words, memorise best moves in certain situations with zero context. Noted. This is why I'm so fricking sick of chess, and why I have only played against bots for the last four months.