Pinning Knights

Noob question here...I'm just gonna ramble out my thoughts on this topic, and I hope I get corrected.

I really don't understand what the point is of using the bishop to pin down knights in openings.  I've heard in a video that a general rule is not to exchange a bishop for a knight in the early game without a really good reason.

I see retaking the bishop does mess up the opponent's pawn structure, though.  But, if I don't plan on trading, then when the opponent moves a rook pawn up one, I just have to move my bishop away...and it seems like having the opponent move a rook pawn up one is a good thing for when he castles on that side since it frees him from back rank problems.

The only thing I've noticed, though, is the abiliyt to sometimes win a center pawn if that knight is a guard.

I'm sure I'm missing things and thining incorrectly in spots.  I'd appreciate any help.

Thanks.

Often it's played without a particular plan when a beginner just wants to tie you down.

But it does serve a real purpose, usually either to weaken the squares the knight is protecting, or possibly to actually provoke the move of the h pawn in an attempt to weaken the King's position, for example. A more obvious reason would be if there is a chance to pile up on the knight to win material, of course. 

One last more subtle situation might be to force a trade for the bishop because either you are ahead material (a good general reason to trade pieces) or if the position after a trade favors knights over bishops -- for example if the bishops are trapped behind pawn chains, or the knights can reach excellent permanent outposts.

All makes sense, thanks.  One question to your response.  You mentioned moving the h pawn weakens the King's position.  How does moving the h pawn do that?

Study of the Ruy Lopez illustrates the ideas behind what you're asking very clearly also.

Moving the h pawn doesn't always weaken the King's position, but often does after you castle short. If your opponent can't get attackers to that side, it's likely not important. 

It's about controlling the center.  Exchanging the knight for a bishop ain't all bad if you get a center pawn out of the deal, too.

At least, i think that's what netzach and echecs are sayin'.  But, I've been wrong before.

That's right BP. Except in the Nimzo-Indian black deliberately exchanges a bishop for knight on c3. In the Ruy Lopez white can exchange for knight if he likes or else makes use of the pin to drag king-side pawns out of position as earlier posts said.

Thanks, I'll check those out.