What if opponent didn't move as the books say

I don't understand, why do courses like to say "White moves A, and Black would moves E, whenever it happens, white could move B.."

what if your opponent doesn't move that way?

like 

White moves e4, black moves other piece.

theory is theory, but your opponent does not have to move that piece.

Books and lessons generally only discuss your opponent's best 2 or 3 moves. It's not feasible to go into detail on all of your opponent's possible moves. Sometimes you just have to think for yourself. With all due respect, at your level you should not study book moves or theory, as your opponents will constantly play random, innacurate moves.

Opening principles not book moves.

It would be nice to show us an example of this. I have a strong suspicion that you misunderstood something.

What to do? Think!

There's always opening principles for beginner and intermediate players such as taking control of the center, developing your pieces to active squares, not bringing your queen out early, and castling. Don't worry about theory. You really don't have to for a long, long while.

Quit trying to understand something that is so far over your head it makes no sense. 

What you are asking is like a 4 year old wanting to learn advanced calculus when they don't even know what 2+2 is. 

Start with opening principles.

Because chess would be a pretty boring game if there was just one line that everyone had to play down every game. 

But yeah, if you’re reading an opening book on a specific line, you’re generally going down one very small rabbit hole and it’s just one of many options the opponent could do. 

I agree with the advice to not worry about it for now and stick with the basic principles. 

Part of learning an opening is understanding why the other moves aren't as challenging. A GM hasn't just memorized moves, they understand the position overall. So they'll be able to play well no matter what move you make.

Generally speaking, understanding an opening in this way isn't possible until you're fairly experienced, which is one reason it's extremely common advice to "ignore openings at your rating" (whatever your rating might be).

Part of what makes chess so incredibly frustrating at lower levels is that people don't play best moves, so many a time you can't follow traditional opening principles. I think the best solution for that is to play higher rated opponents for a long while and then when you're confident about your abilities, make a new account and start from say... a rating of 1300-1400, when idiotic play is less frequent.

I have a similar problem with the Lessons page.

I don't doubt their usefulness. But the positions they provide are positions that I will never be able to take advantage of because I am never able to reach those positions.

What level lessons are you doing?

Hi! My name is Lauren Goodkind and I’m a respected  chess coach and chess YouTuber based in California: 

https://www.youtube.com/channel/UCP5SPSG_sWSYPjqJYMNwL_Q

Here’s an idea to help you get better.  

-Consider all checks and captures on your side and also your opponent’s side. Always as, “If I move here, where is my opponent going to move?”. Do this for every single move!  

-If you are talking about the opening, then focus on developing all your knights and bishops out toward the center.  

I hope that this helps.  

 Oh, if only that were true...

In this case you start playing chess.

EXACTLY!!!!

@hrarray, that is a good strategy. If they don't play a book move, there is probably a reason WHY the move they play is not a book move. Maybe also you could study these other moves when you study your games.

No they don't. Most of the books on the sicilian are like: Ok so starting at our normal position after 10 moves of theory, here are the 10 things white could play. We will go over each one.