You've found some text that is perfect for your paper. You can't think of how to summarize or paraphrase it while you are writing, so you copy and paste the entire passage into your paper. A few days later, you are re-reading your work, and remember that this section isn't your own writing. But you don't remember which source it came from...
Simply swapping out words or phrases in copied text is considered plagiarism. To avoid this, make sure to copy and paste the URL/link along with the passage as a reminder to go back and integrate the source at a later time. You always need to cite sources that you summarize or paraphrase.
Some library databases have built-in plagiarism prevention tools. When you copy and paste text from an article or eBook in a library database, a formatted citation is included with the text so you don't forget where the information came from.