I love Norm Lewis’s thinking on this. He says there’s little use in getting hung up on intent: Whether a writer meant to plagiarize or not is a question that should be weighed when determining the severity of the crime, not whether it occurred.
Anyway, here’s the flowchart and the Poynter column by Benjamin Mullin that should help you stay out of trouble when writing stories. In my experience, reporting students get into trouble in two ways: 1) by cutting and pasting from online sources directly into their own work without attribution and 2) lifting facts from other news sources and using them in a story as original reporting. Example: Say you can’t get the name of the suspect in a crime, but some local TV station reports it. You can’t use that name in your reporting without attributing it to the TV station. (And by the way, it would be dumb to use it without obtaining official confirmation because all news sources are wrong sometimes.)
And as always, if you have any questions about what to attribute in a news story, ask your editor.