Writing the introduction after writing the body paragraphs might seem a bit backwards to most people but the introduction is just the statement of what you're going to discuss in your body paragraphs so it makes sense to write those first.
As I'm writing just a section of a chapter in a book the introduction and the conclusion will have to be about the entire chapter and not just my one section. Which means that I will have to discuss the other topics in the chapter with the person writing about them in order to write a proper introduction and conclusion about the full chapter.
The purpose of an introduction is to give the reader more contextual information of your topic so that they can understand the analysis that you make throughout the paper. It should state the scope of your review, give your objective and tell the reader how your review is organised.
To write a good introduction you should make sure to avoid giving too much detail on whats in the rest of the paper, you want to hook the reader, highlight the importance of the topic that you're discussing in your paper, try to convince the reader that even if this is something they're not particularly interested in that its something they should still read.
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The purpose of the conclusion is to summarise everything you've said in your paper so far for the lazy reader who didn't want to read more than one hundred of the words on the page. The summary should include your key themes, the overall findings and the relevance of the topic to current knowledge.
In case your introduction didn't work and your reader wasn't hooked, this is your second chance. If the reader has skipped the body paragraphs to read the conclusion the summery of your paper and your topics should hopefully be able to convince them to go back and read what you've written .