It is not always easy to separate fact from opinion. Skilled writers can make you think their interpretations are facts. Assumptions should be reasonable. Note errors or omissions. You should explore enough sources to obtain a variety of viewpoints.
Primary sources are the raw material of the research process.
Secondary sources are based on primary sources. For example, if you were researching Adenauer's role in rebuilding West Germany, Adenauer's own writings would be one of many primary sources available on this topic. Others might include relevant government documents and contemporary German newspaper articles. Books, encyclopedia articles, and scholarly journal articles about Adenauer's role are considered secondary sources. Choose both primary and secondary sources when you have the opportunity.