Are you allowed to copy a pattern? Legally NO. To the best of
Are you allowed to copy a pattern? Legally NO. To the best of my knowledge, the fine is ten thousand dollars and or ten years in prison.
So, please DO NOT ask your local yarn shop to disobey the law.
What is not so clear is the ability of an individual to make a copy of a pattern they have purchased. Apparently it needs to go to trial for a precedent to be set. It is generally assumed that you can make a copy for your own use, just beware that this is a legal gray area.
I am hard on patterns; shoving them in bags, spilling liquids, tearing, writing notes on them, highlighting, folding/spindling/mutilating, and even losing them. In this image I have highlighted each row as I finished it on a copy. This really helps me on complicated patterns. (This is the pattern used for the shawl whose border is shown in the Swatches (Gauge) article.)
The original is safely stored unmarked should I need a new copy, want to knit the same item again, or knit another item at the same time that is included in the pattern book.
What you can do is lend someone your pattern when you are finished with it, just as you can when you finish a book.
Can you use a pattern for commercial purposes? Mostly the answer is no but the pattern may grant you certain rights and should say so clearly. If in doubt, contact the author of the pattern.