Simple:
* The historian relied on secondary sources to learn about the event, as no primary sources were available.
* Secondary sources, like textbooks and biographies, provide a broader overview of a topic.
More specific:
* Secondary sources, such as scholarly articles and reviews, helped me analyze the primary data I collected for my research.
* To understand the context of the painting, I consulted secondary sources like art history books and museum catalogs.
With an example:
* My essay on the American Revolution used secondary sources, like David McCullough's book "1776," to understand the key events and figures.
Remember to choose the sentence structure that best fits your context and the overall tone of your writing.