Each of the rows on the keyboard is a single type of attack:
The further right key you press, the less stamina it takes, but also the longer the delay.
Balance catching your enemy off guard with saving enough stamina to block.
Make a high attack against a low block, and a high block against a high attack (etc).
Blocking at exactly the right moment staggers the enemy, reducing his damage for a while.
You can stack the same attack in the same square, dealing more damage and staggering when it hits.
Damage takes away your block stance (which is the blue icon that sometimes appears in the upper left).
Leave empty turns to recharge your stamina or you will be vulnerable to attacks.