No disrespect but your question is an example of why beginners are often told to forget about learning openings in any detail, just to concentrate on opening principles.
You should always play your move based on the position of the board in front of you. That goes for the opening moves as well as the endgame.
Learning a set of opening moves without understanding their significance is no way to learn chess. At casual play level openings go off book very quickly and you will be lost just as fast.
I did not learn any openings yet but I want to at some point...and of course I understand that when learning an opening you have to understand the reason beside each move.
When your opponent goes out of opening book, you get to start playing chess!
Answer: Smile and enjoy yourself.
"... there will come a time, whether on move two or move twenty, when your knowledge of theory runs out and you have to decide what to do on your own. ... sometimes you will leave theory first, sometimes your opponent. ... It happens in every well-contested GM game at some point, usually a very significant point. ..." - IM John Cox (2006)
"... For beginning players, [Discovering Chess Openings by GM John Emms] will offer an opportunity to start out on the right foot and really get a feel for what is happening on the board. ..." - FM Carsten Hansen (2006)
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf
"... Overall, I would advise most players to stick to a fairly limited range of openings, and not to worry about learning too much by heart. ..." - FM Steve Giddins (2008)
"... Once you identify an opening you really like and wish to learn in more depth, then should you pick up a book on a particular opening or variation. Start with ones that explain the opening variations and are not just meant for advanced players. ..." - Dan Heisman (2001)
https://web.archive.org/web/20140626180930/http://www.chesscafe.com/text/heisman06.pdf
"... To begin with, only study the main lines ... you can easily fill in the unusual lines later. ..." - GM John Nunn (2006)
"... I feel that the main reasons to buy an opening book are to give a good overview of the opening, and to explain general plans and ideas. ..." - GM John Nunn (2006)
"... If the book contains illustrative games, it is worth playing these over first ..." - GM John Nunn (2006)
"... the average player only needs to know a limited amount about the openings he plays. Providing he understands the main aims of the opening, a few typical plans and a handful of basic variations, that is enough. ..." - FM Steve Giddins (2008)
"... For inexperienced players, I think the model that bases opening discussions on more or less complete games that are fully annotated, though with a main focus on the opening and early middlegame, is the ideal. ..." - FM Carsten Hansen (2010)
"... Everyman Chess has started a new series aimed at those who want to understand the basics of an opening, i.e., the not-yet-so-strong players. ... I imagine [there] will be a long series based on the premise of bringing the basic ideas of an opening to the reader through plenty of introductory text, game annotations, hints, plans and much more. ..." - FM Carsten Hansen (2002)
https://web.archive.org/web/20140627055734/http://www.chesscafe.com/text/hansen38.pdf
"The way I suggest you study this book is to play through the main games once, relatively quickly, and then start playing the variation in actual games. Playing an opening in real games is of vital importance - without this kind of live practice it is impossible to get a 'feel' for the kind of game it leads to. There is time enough later for involvement with the details, after playing your games it is good to look up the line." - GM Nigel Davies (2005)
Let's say a game starts with an opening and on move 5 opponent steps outside of opening book? Should you continue playing some moves from the opening book or make your moves based on how his last move affects the game as you see it?
The latter
I actually learned that strategy from Bill Belichick
Let's say a game starts with an opening and on move 5 opponent steps outside of opening book? Should you continue playing some moves from the opening book or make your moves based on how his last move affects the game as you see it?