What do you do if opponent doesn't respond as opening suggest?

I'm looking at different openings and I notice that every opening assumes that the opponent will move a particular way. What do you do when your opponent doesn't do as intended. Even when I use Chess.com's computer anaylsis, the computer assumes that opponent would have done a certain move. Do you contine on with the opening anyway? Thank you.

If your opponent plays a different move not in your line you have to use general opening or chess princeples, This would be good place to take some extra time on this move and try and figure out the advantages and disadvantages. Also most openings have common themes that you can follow. 

If all you are doing is memorizing lines, you are are completely wasting your time.

Being able to parrot lines makes you what?  As smart as a parrot?  Bird Brain!

The key is understanding the underlying reasons behind each move.  If you aren't truly UNDERSTANDING the reasons behind each move, what good are ya?  ZERO, that's how good!

After 1.e4 e5 2.Nf3 Nc6 3.Bb5, why does Black play 3...a6?  Doesn't this just drop a pawn?  White can take on c6 and then take on e5.  Problem is that after 3...a6 4.Bxc6 dxc6! 5.Nxe5 Qd4!, Black will get the pawn back with Interest.  It isn't until you understand this Tactic, and understand that you are forcing White to either take all pressure off the Knight on c6 (by retreating on the f1-a6 diagonal), trade Bishop for Knight (via Bxc6) or most common, retreat the Bishop to a worse diagonal, but maintain the pressure on the pinned knight and e5 pawn.  Then, after 3...a6 4.Ba4 Nf6 5.O-O Be7 6.Re1 b5 7.Bb3 d6 8.c3 O-O 9.h3, Why does Black often go to the rim with 9...Na5?  Stuff like this is what you must understand, not just being able to parrot a bunch of moves.

You need to get out a 3-D Chess Board, put it in front of you, get out the pieces and your opening book, and study.  It's the only way to get better and to truly understand the ins and outs of the opening.

You play chess. This is why technique is more important than opening theory at the amateur level.

This is why beginners are advised not to learn opening "lines" but "principles"

I can never remember lines anyway.

Dang, it sounds like stuzzicadenti has been reading my posts on this topic.   He said exactly what I was going to say. A move that is out-of-book is probably:

(1) suboptimal => you need to know the position, ideas, and goals to know why, and figure out why it's suboptimal to exploit it:

(1a) an outright tactical mistake => figure out why: does it allow a fork? stall too long  before castling? leave a piece en prise?

(1b) a positional mistake => there is no immediate refutation, only a lost tempo or poorly placed unit that will inevitably cause him problems in the long term

(2) a Theoretical Novelty => if prepared by a GM, you'd better be an expert in the opening to be able to refute it over the board; if prepared by a class player, just know your opening well enough to refute it over the board

(3) an innocent transposition => just play the move you would normally play, especially if it's a semi-open defense like the French, Caro-Kann, or Pirc where the move order tends not to be very important.

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(p. 1)
   An apt illustration occurs in
deviations from "book". A game
begins with 1 e4 f6. The reply is
bad, so bad in fact that it will
not be found in any collection of
standard opening moves. What to
do about it? The man who has
memorized oodles and oodles of
moves without understanding
them is at a loss; he will not even
be able to give a good reason why
the move is bad. But the man who
knows that Black has neglected
the centre, deprived his KN of its
best square, and weakened his
King position will find it a simple
matter to refute his opponent's
faulty play.

Fine, Reuben. 1989. The Ideas Behind the Chess Openings, Algebraic Edition. New York: Random House, Inc.

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(p. 16)
   Some players make their opening
moves confidently, but collapse
when the game gets 'out of the book'.
So our final piece of advice is not to
make a hasty reply when your
opponent makes an unexpected move
in the opening (or indeed at any
time), even if it appears very weak or
very strong; remember that 'obvious'
checks and recaptures are not always
the best moves. The first step is
always to work out what your op-
ponent is threatening, and to see
whether you have any threats that he
cannot meet. Three or four 'candi-
date' moves will probably suggest
themselves to you, and you must next
analyse and evaluate these. Even if
your opponent's new move is 'the
latest Russian improvement', this
methodical approach will always
provide you with a mental buffer and
give you a good chance of finding the
best plan in reply. Even at quite fast
time-limits, it is worth spending ten
minutes at this critical stage when the
opening becomes the middle game.
And it is only by thought taken in the
true testing-ground, serious play, that
you will obtain real understanding.

Harding, Tim, and Leonard Barden. 1976. Chess Openings for the Average Player. Mineola, New York.

Another point to mention is to be patient. If your opponent plays an unconventional move that books don't mention and that looks whacky, the move probably is inferior, but maybe not as bad as it looks. I have lost games by getting annoyed by an eccentric opening move and playing with unwarranted aggression thinking I should "punish" my opponent within 20 moves. That attitude can lead to disaster.

I use the opening principles, and study the pawn structure.

I don't think it's a waste of time to learn openings.  If your opponent deviates from the mainlines these lines give you a point of reference provided that you understand them.  Compare your opponents moves to the "book" moves to get an idea of their relative strengths and weaknesses.  Based on an analysis you might continue with the usual plan or you might need to adjust your play to capitalise on the weakness of your opponents move. 

One very important thing, alot of people like to blitz out their opening moves.  This is fine as long as you're in the book, however I've noticed many players have a tendency to try and keep playing quickly even after they are de-booked.  As soon as your opponent plays something unfamiliar STOP, and have a good think about what their move achieves and this will help you develop a plan for how to continue.

ThrillerFan is exactly right. Memorizing is okay if you understand the reason for each move. For example, if the bad guy was supposed to move his Knight to f3 according to the book opening, there was probably a reason for that. The Knight may have needed to go to f3 to guard another piece, like a Bishop. Since you know this, or should know it, when you see the Knight goes somewhere else the you know it is not doing its intended job or guarding the Bishop. You can maybe go after that unguarded piece (the Bishop). This a simplified example. Does that make sense?

Then you start playing chess.

study 8-9 openings and defences in depth! it will take years. every time you win or lose gi back and find the move where the game wsd eon or lost. Main lines normally last 8-10 moves and eventually you will be off the main lines and you'll need to use your wit and tactical nous to win.

If their move is not "in the Book" then there is a strong chance it is weak so start looking at what weaknesses it may be creating. As others have suggested know the basic principles behind the opening, this will give you some clues as to why a move may be weak, then you can work out your response to it.