Question: If you take and/or copy another persons work and call it your own but make changes to it is that illegal?
This answer kind of depends upon what you mean by work. If the 'work' that you're copying is a book, a movie, a web-page or something published like that, then using it and calling it as your own would be an violation of copyright laws.
Copyright is a legal concept that comes from federal laws. A copyright means that when someone creates a 'work', they own that work and other people can not copy it without their permission. If you take a work and make changes to it, this is called creating a 'derivative work' and it is still a violation of copyright laws.
Now, if the 'work' in question were something like a 'term paper' that a classmate wrote (although this could qualify for copyright protection, the author probably would not have applied for copyright protection), then we would normally call this 'plagiarism'. Plagiarism means taking someone's work and calling it your own. While this isn't typically illegal, it is always unethical and will be a violation of most schools' codes of conduct. If you do this and get caught, you will likely be expelled or suspended from school and might receive a failing grade for the assignment.