To ensure that the structure of the text is solid, the paragraphs, too, have to be well structured. A paragraph typically consists of three elements: a topic sentence, supporting sentences, and a concluding sentence:
1. In the topic sentence (introductory sentence), the topic or focus of the paragraph is presented. The topic sentence serves as a focal point, foregrounding the content of the whole paragraph. By signalling to the reader what the paragraph deals with, the topic sentence will thus increase the readability of the text. Although it is possible for the topic sentence to appear anywhere in a paragraph, it usually appears at the beginning.
2. The main part of the paragraph consists of supporting sentences: this is where the argument that explains and/or proves the topic sentence is delivered.
3. At the end of the paragraph is the concluding sentence (transition sentence), which sums up the argument of the paragraph, and may create a transition to the next paragraph. A transition provides the text with a smoother flow between paragraphs.
Instructional video from the free online MOOC "Writing in English at University" which was developed at Lund University in 2016.