My name is Mia Oscar and I am telling about you how to write a topic sentence. When writing a non-fiction essay, the first thing to keep in mind is that you must be very clear about your topics. For example, for a research paper on deforestation, your topic could include the reason for deforestation. You could then go on to say how it affects the ecosystem as well as the local communities. You can then go about explaining in detail all other aspects of deforestation. The simple rule is: write about only one topic per paragraph – but there could be more than one point in each of those paragraphs or sentences.