Languages change due to the needs of the speakers as new technologies and experiences require them to use new words to communicate effectively based on their situations. Secondly, the English spoken today is different from the one spoken earlier due to experiences from one person to another.
Primary sources are usually those written from people who experienced that event and/or provides evidence such as an item of clothing or even an oral account of explanations. An example of this would be Anne Frank's Diary or A pillar engraved with hieroglyphics.
