How to play an opening verses erratic opponents?

Hey everybody I'm only a beginner at chess. I only learnt how to play not long ago.

I want to learn how to play effective openings like the italian game, the gambits, sicillian defence etc but my problem is that when I verse opponents, they never take the theory lines.

For example, I play very often against my mate however most of the time he will play a weak move against me. How can I play an opening when the move he makes is not included in any often played line because it is not a very good move! In saying that it's a "bad" move, it's still not bad enough to cripple his defence or allow me to exploit it!

For example, regardless of me playing either e4 or d4 he will always attempt to fianchetto his bishops, every game. How can I play most effectively against this?

Thanks, all advice much appreciated

well, you almost answered your own question.  consider this:

if you know the well-known line in your opening, and have reason to believe it's played like that because they are the best moves, and your mate doesn't play them, well, then, as you said, they are weak.

you have to find out why. why are those moves weak? why does it not further the theme of your opening? why does it place his pieces in a worse position to carry out a plan than yours? why did he *make* that move, in the first place?

once you figure out 1. why the move is bad and 2. how to bust it, you are well on your way. 

Most small opening inaccuracies take master level play (or better) to exploit.  For example finachettoing his bishops in the opening is not something you can refute or "bust"

Claim some center space, develop your pieces quickly, and castle, that's all you should worry about in the opening right now.  After you've done those things if he's behind in development then try to open lines.  If his development is fine, then it's a game of chess (as they say).

IMHO it's a common misconception for newer players that you can essentially refute 99% non-book moves, I'm glad you saw that's not the case here :)  The difficulty is opening inaccuracies aren't punishable in the immediate.  To punish something that happened in the opening, most often you'll have to skillfully massage it into a superior middlegame position and that's not a simple matter -- more like years of practice / experience :)

To an IM title is the end of that thought. 

I'm all for thinking through an opening, but I don't think our man here is going to bust his opponent for double finachettoing in the opening if you see what I mean.

Thanks for the response Skwerly. In response to your comment:

Well I know, for example, that he plays a regular fianchetto because he likes to have his bishops control the diagonals. Conversely, I understand that by playing g7 or b7 as his opening moves (as black) he is giving up control of the center of the board and decentralising his attack.

Whilst I understand that is is detrimental to one's game to relinquish control of the center of the board, I do not know why this is OR how to punish a player who doesn't do it.

Could any experienced hands point me towards some resources which would address facets of the opening such as these? I flicked through opening theory for dummies but once again, it only showcased the opening lines when the opposition assumed the optimal line, not when my opponent plays random, haphazard moves.

Thanks!

@wafflemaster

Thanks for your input also. I didn't realise that the answers I seek are infact master level questions. I guess it makes sense though, because if you understand every facet of a move and why it was a good/bad one, then you can play the absolute most efficient move on your next turn. 

I guess what you are saying is that, in response to a weakened but netherless solid opposition's opening, the best I can do is develop my pieces, control the centre, castle early, and then work on launching an attack? If not feel free to correct me.

Thanks!

In the 1920's a strong player nammed Nimzowitsch championed the hypermodern style of chess which was to challenge the center from a distance with pieces, and not occupy it with pawns.  The idea is to let your opponent establish a large center and then try to prove it's a weakness that's subject to attack.  Which is to say, don't think a pawn in the center is the be all end all of the opening :)

Despite it's gimmicky look, fianchettoing both bishops isn't weak.  It's not that great, but it's not so bad either.  That said you should maintain a reasonable edge if you go for something like this:

Your opponent has no weaknesses to focus on, so you're just continuing to develop.  The general idea is that eventually his cramped position will tell as his defensive options are diminished.  This handicap is often exploited by building threats on one side, then shifting your pressure to the other side... your space allows you to alternate threats, his lack of space does not allow him to shift defense.

Common error?  To assume you're winning.  It's a nice edge, but don't get cocky.  Always watch for tactics, and keep the pressure on without trying to force matters... success means you'll see a clear continuation to win some pieces.  Again don't try to force what's not there.  If it's not there he's likely only escaped with equality, if you over press then he'll be the one with an edge :)

Oh wow, do I feel like an ass!

Most of the time I will avoid playing that position as white because I thought there was weakness within symmetry. I guess that's out the window!

The play that you have assumed as black is a common opening that he would adopt indeed. So once established, control of the centre allows alternation between attacking flanks and hence capitalises on his weakened position resulting from his cramped defence/ lack of development. I will remember all of this and try to apply it to my game.

Thanks!

No problem :)

If he does it no matter what, and basically ignores your moves, you could go for something more agressive like this :

there is no weakness in symmetry,.. in fact you can learn a lot from playing positions that have it. White generally gets a small edge because they can break the balance first. but if white takes to long to do something black can counter attack... the sicilian looks for an imbalance right away.