Good job! In looking at these examples, you have identified several common concerns about plagiarism. To summarize:
Turning in someone else's paper as your own work is plagiarism.
Quoting someone else's writing without giving credit is plagiarism.
Material taken from the web should be cited, just like any other source.
"Common knowledge" does not have to be cited, but if you are in doubt that something is common knowledge, cite it anyway.
You must cite the ideas of another person, even if you did not use their exact words in your paper.
You're probably pretty clear now on how to avoid intentional plagiarism. But not all students who plagiarize do so on purpose. Sometimes plagiarism is the result of carelessness during the research process. Here are some strategies that will help you achieve several goals:
get the most out of the research process;
improve your academic writing skills;
and avoid "accidental" plagiarism.