Shadow defense

I'm having trouble with this lesson http://www.chess.com/chessmentor/view_lesson?id=6265 ... all fine against the mentor, but when I play it against the computer I always end up losing, typically as above. Any ideas?

EDIT: Sorry... I'm new to this site, I can't get the PGN editor to work; this is the PGN I was referring to 

[Site "Chess.com"]

[Date "2014.03.13"]

[White "Computer - Hard"]

[Black "Player"]

 1. ---- Ka8 2. Kd8 Kb8 3. Ke7 Ka7 4. Kf8 Kb8 5. b5 Kb8 

Hi emilio89--there are two options for black here: the one that's tempting but wrong is to chase down the b-pawn.  After ...Kxb4 Kxc6 in a few moves, black has no way to circle around to stop queening on d8.  Black draws with "distant opposition," meaning he keeps his king an odd number of spaces away from white's in a direct line.  In this case it's three squares, so 1...Ka8 is the only correct starting move.  Direct opposition with 1...Kc8 loses after 2. Ke7 Kc7 3. Ke6 because black's king gets shoved away from his c-pawn.  He needs to avoid his own pawn UNLESS white plays Kd6, in which case he must be able to immediately answer with ...Kb6.  Then Kd5 is impossible, Kd7 is answered with ...Kb7, and white has to step away again.  This is the only drawing method: keep a distance until white approaches on d6, then move into b6 and force him away again.

Hi, thanks for your answer! I understand the chess mentor lesson, but I got stuck when playing against the computer because he surpised me with the b5 pawn push. I always kept the opposition, but after

Ka8 2. Kd8 Kb8 3. Ke7 Ka7 4. Kf8 Kb8 5. b5

if I simply take he queens with the other pawn on d8.

That's true, so you play 5...Kb7! instead.  White's d-pawn can't move, it's lost if he takes on c6, and otherwise you both queen without losing a turn to check now that your king is on the 7th rank.  That position is drawn.

Ah yes, hadn't thought of that! Thanks a lot, you've been a great help