First sentence is your topic sentence. When you wrote your essay you began your essay with a thesis encapsulating an overarching idea that can be broken down into three sub-ideas that all synthesise together to form the foundation of your thesis. The topic sentence basically outlines one of those sub-ideas. The second sentence varies among students, schools and markers but I often use it to be an overarching ‘context’ sentence that explains the topic sentence in the context of the historical paradigms / social values / authorial context of the text, linking it to the author’s purpose and back to said sub-idea.