Can Someone Explain This Mistake Please?

I don't know how to copy/paste games here, so I took a screenshot from the game review I am doing.  I am playing black -- Opponent is a bot -- on my 6th move I placed my dark-squared bishop on B4.

This was my thought process. 

• This is a good move because I am attacking white's knight.
• This is a good move because white's knight is now pinned to the king.
• This is a good move because it prepares me to castle. 

FWIW, I won the game on the 15th move and thought I played a good game. When I did the game review, I was surprised to see that the move illustrated here was considered a mistake, and that -- according to the game review -- I had lost my winning advantage. A better move would have been moving my knight to b4. Why?

Can someone please explain -- in simple beginner terms -- what was really wrong with moving my bishop? I'll be glad to share the PGN file here if someone can tell me how to do it.  

Thanks so much for any help on this. 

Thanks for posting it for me. I appreciate it. 

Good! I'm doing my best to think through my moves and play very carefully.  

I'm no expert, but I think the Game Review counts this as a mistake because white can defend the knight easily with a normal developing move (Bb2 or Bd2), whereas if you'd put your knight on b4 instead, you're threatening to win the c2 pawn outright (possibly with a fork).

Here you go.

It isn't that the bishop is a terrible move on its own its just white can meet the pin threat whereas if you jumped in with the knight with the threat of a fork, white has a harder time defending that. The knight move would've made whites play incredibly passive at best.

Oooohh... yes, I see that the knight could take the pawn and fork the king and the rook! I'm learning to look at each move more carefully, but I haven't got the ability yet to look ahead to the next move. Thank you so much for pointing this out.

Thank you! I was focusing more on d7 instead of c7. I see the weakness there now. It really helps me to have explanations like this. Thank you again.

Thank you. It's making much more sense now. 

You're welcome. 

Thanks! Appreciate the support.

You won against Azeez! Well done. Did the analysis offer a "best" move? I would also have gone with the knight, but bishop not bad.

Hi Luke, I'm not Japanese, I'm Australian but I live in Japan (Gunma). 19 years now.

Yes, the analysis suggested the knight move...and at first, I didn't understand why. Now I can see why it would have been a better move. 

OP I thought your victory when you were down -12 was pretty neat.  You have a fighting spirit! ✊

Thanks... it's been brutal morning. Really, really bad. But I'm trying.  

Nb4 gains an attack on c2 that's impossible to defend because of the LSB!
Seeing these power points is a necessary skill to learn at your level.
Whenever you get an attacking chance like this, it should get all your priority over development, king safety etc. The bigger picture is often that Checkmating patterns > Tactical patterns > Direct Attacks > Strategic positional development > piece development > king safety > space advantage. Danger levels!

Trying to figure out priorities has been tricky. I know I need to develop pieces. I know I need to castle early.  But I don't know when other things take precedence over those. I'll just keep at it!