Distinguish primary sources -- firsthand accounts and original documents -- from secondary sources, using a real Revolutionary-era letter as an example.
A primary source is a firsthand account -- something created by someone who was actually there: a letter, a diary, a speech, a photograph, an original document. A secondary source is created LATER by someone describing or analyzing what happened -- a history textbook, a documentary, an encyclopedia article.
Primary sources put you as close as possible to the actual moment, in the actual words of someone who lived it -- but they also only show ONE person view, which is why historians use many primary sources together, plus secondary sources, to build the fuller picture.
Mastery looks like: They correctly classify sources as primary or secondary and explain the reasoning, including the value and limits of primary sources.
Common stumbles: Assuming "old" automatically means "primary" -- an old textbook can still be a secondary source if it was written after the events it describes.