>>8 is true in the world of Academics, where you have to reference someone else's work if your work/conclusion is similar, even if you've never heard of those works prior to the conclusion of your works. Mmm, delicious time paradox...

>>1 shouldn't worry too much though. In order to learn, you'll need to learn some by copying. But many times, if you're gonna write something bigger, it will be too much of a hassle to copy different sources, and you'll write your own code to get it done "your way".
Though there's also sometimes when one chooses to use modules of other people's more complex pieces of code within one's own code - i'm not 100% on the legality, but morally you should only do this if you have legally aquired rights to do so, for instance if it's been declared that people may use it by the creator, or possibly that you've purchased the rights to use it, though I have zero experience with buying code. Remember, if you do use bigger chunks of important code, you should reference whose code you used. If not for anything else, for people to know whom to throw the blame on in case that piece of code turns out to be a nest of bugs.

I use what's out there for free, or make my own. Most of the time I'm better off coding from scratch.