Responding to unorthodox opening/defense ?
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I ask this because, so often I'm reading that I should learn an opening and play it over and over so I can learn it inside and out, before adding an additional opening to my repertoire. But, this proves difficult when the people I'm playing against are all over the map with their responses.
If your opponent's move went away from the theory, U should estimate, first of all, whether it represents any danger. If that move is more or less meaningless (it will happen often), U can continue to develop your pieces as planned. 
Perfect. Thank you!
Usually the most popular move is the most popular for a reason. If a random move threatens something you must deal with it. If not try to understand why a certain move is usually preferred in the position and 'punish' your opponent for not making it eg. Are you now able to make an annoying pin with a bishop or win a tempo by attacking something with a knight which the most popular move would have prevented? If not then you can just follow your original plan and will end up with a better version of the 'normal' position because your opponent played something irrelevant.
I like that. Thank you! And, your profile name alone convinces me you know what you're talking about.
Jengaias, thank you!
Here is the thing. There is an idea, a general plan behind every well-developed opening. When you are playing Italian game, you are not playing it to pass a high school course, you are playing it with some plan in mind. If the opponent deviates from the main line, then you should still go on with your plan, adjusting it depending on what the opponent plays.
For example... When you play Sicilian Defense, what you likely want to get out of this is a strong counterattack on the queen side. So, if you see 1. e4 c5 2. Nf3 Nc6 3. d4 cxd4 4. Nxd4 e6 5. h3?!, you see that this move doesn't really affect your plan and you are free to continue with it by, for example, 5... a6 and developing your pieces similarly to how you do it when your opponent plays by the book.
What is your general idea when playing Italian Game? What are you trying to accomplish positionally by playing it? Whatever your goal is, unless the position warrants changing it, keep moving towards it. The theory is there just to guide you, it is not something you should follow blindly.
Hope this helps.
"... Throughout [Discovering Chess Openings] Emms uses excellently chosen examples to expand the readers understanding of both openings and chess in general. Thus equipped the student can carry this knowledge forward to study individual openings and build an opening repertoire. ... For beginning players, this book 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, reviewing the 2006 Emms book
https://web.archive.org/web/20140627114655/http://www.chesscafe.com/text/hansen91.pdf
MayCaesar and kindaspongey, thank you both for your excellent and highly appreciated responses! 🙏🏼
You may also be interested to check out a few of the books specific to opening play in the following list of books for those who have not yet added "Master" to their chess title. Especially appropriate at this point in your development would be John Emms' "Discovering Chess Openings", as has been previously recommended - it's an excellent introduction, specifically focused on fundamental principles of opening play...
Good Chess Books for Beginners and Beyond....
https://www.chess.com/forum/view/chess-equipment/good-chess-books-for-beginners-and-beyond
RussBell, thank you!
QUESTION:
If you are, lets say white, and you open with king's pawn, and things begin to develop into a situation where you could play, say the Italian Game, and as all is appearing mainline, black throws in something from way out in left field, like h5 and then a release of his rook, or a development of their queen much much earlier than expected (though it's not necessary attacking just yet), do you just continue to develop as you would anyway, or do you now have to completely change up your opening plans and continue the remainder of your opening by making developments in response to potential moves from black, thus losing tempo?
NOTE:
It might be good to note that, because I'm a beginner with a daily score of 1000, I'm playing others in that range. Therefore, I'm inclined to suspect that the curveball moves I'm inquiring about are likely not made on purpose to throw me off, but rather as a natural response from my opponent, believing it to be the best move with no specific foresight in mind.
Any constructive thoughts or advice are greatly appreciated!
Thank you