Plagiarism is usually unintended.
The practice of copy/pasting content you found online or from a book and/or journal can be problematic. Try the following to help you avoid plagiarism:
Recognizing and Avoiding Plagiarism Exercise
Aiming for Integrity: How well do you know plagiarism? (Take the Quiz)
Resources to Help Detect, Prevent and Avoid Classroom Plagiarism for Teachers and Students
How to Recognize Plagiarism: What you should do to avoid plagiarism you must give credit when...
How to Recognize Plagiarism: Tutorials and Tests
APA Blog: Principles of Good Writing: Avoiding Plagiarism
Copyright Law of the United States
Plagiarism: Citing Sources and Avoiding Plagiarism -
Princeton University - Examples of Plagiarism -
The George Washington University Writing Center
When U.S. Works Pass into the Public Domain