>>1
The way to do it is to look at the code, understand how it works, then /put it away before you write your code/. Then you can honourably claim that you wrote the code yourself, even if you got the algorithm from somewhere else. You should probably cite where you got the algorithm from.

A technique that's helpful for doing this is to keep an editor window open, and paste in every source you see that have code you think is useful. That way you won't forget anything when the time comes to do your bibliography; you can just look back over the list.

Finally, copyright only resides in creative works. The thing is, if there's only one sensible way to do something, then this implementation is not creative, and not copyrightable. So it doesn't matter if you copy/paste or rewrite by hand an uncreative element of a work. But copy/pasting other people's code is a bad habit, and you probably shouldn't do it anyway.

For example, " f = fopen(filename,"r");while(s = fgets(f)){} " is not copyrightable, but, for instance, if the core of the while loop uses a regexp, or nested conditionals, or whatever, there's probably creativity in there. This is somewhat tangencial to the issue of plagiarism, but probably still valid.

>>12
That would be perfectly acceptable. But an essay that's entirely comprised of quotes would get no marks. And probably infringe copyright, but that's a completely separate issue.

>>17
If you claim that you wrote it, then yes you are. If you don't, then no you're not.