why was Pip so suprised when he hit the pale young gentlemanthe first time? why was he suprised again the second time

Pip thought the boy could fight because of the way he danced around. He never expected to actually hit him, let alone knock him down and bloody his nose.

"I never have been so surprised in my life, as I was when I let out the first blow, and saw him lying on his back, looking up at me with a bloody nose and his face exceedingly fore-shortened."

"But, he was on his feet directly, and after sponging himself with a great show of dexterity began squaring again. The second greatest surprise I have ever had in my life was seeing him on his back again, looking up at me out of a black eye."