Don't use another's work. EVER.
The whole all my own work thing may be a joke and all but DoE is willing to take measures if you get caught.
I handed in an essay once and the head teacher deducted marks for plagiarism, "because she did not expect a year 10 student in a school ranked below 500 to be able to write an essay better than her senior english class". And she didn't even run it through the checkers.
Another story: most of my commerce class in year 10 decided to try copying little snippets of info from a booklet our teacher had given us to revise.
Most of it was only a few words here and there, but I guess it was all a set up.
Out of just under 30 students, less than 5 students actually received a mark at all. Most scored zeroes for copying things.
TL : DR Don't copy anything, ever
Edit: Learning how to do something yourself is the best way to learn. Sure, you can score a mark easily in the short term, but come HSC time, and you've got no idea on the whats, whens, whys. Analyzing someone else's is, as above been mentioned a good way to grasp the concept quickly, but don't copy it. Understand it then make it yours.
GL HF o7