In many languages, the fundamental unit of composition is the paragraph. A paragraph consists of several sentences that are grouped together. This group of sentences together discuss one main subject. In formal academic English, paragraphs have three principal parts. These three parts are the topic sentence, body sentences, and the concluding sentence. We will also talk briefly about details in paragraphs.
The Topic Sentence
A topic sentence usually comes at the beginning of a paragraph; that is, it is usually the first sentence in a formal academic paragraph. (Sometimes this is not true, but as you practice writing, please keep to this rule unless you are instructed otherwise.) Not only is a topic sentence the first sentence of a paragraph, but, more importantly, it is the most general sentence in a paragraph. What does "most general" mean? It means that there are not many details in the sentence, but that the sentence introduces an overall idea that you want to discuss later in the paragraph.
When a reader reads a topic sentence a question should usually appear in the reader's mind. The reader should then expect that the rest of the paragraph will give an answer to this question.
That is, the second and third sentences give some explanation The second and third sentences are called supporting sentences. They are called "supporting" because they "support," or explain, the idea expressed in the topic sentence. Of course, paragraphs in English often have more than two supporting ideas. At minimum, you should have at least five to seven sentences in your paragraph.
The Concluding Sentence
In formal paragraphs you will sometimes see a sentence at the end of the paragraph which summarizes the information that has been presented. This is the concluding sentence. You can think of a concluding sentence as a sort of topic sentence in reverse.
You can understand concluding sentences with this example. Consider a hamburger that you can buy at a fast-food restaurant.* A hamburger has a top bun (a kind of bread), meat, cheese, lettuce, and other elements in the middle of the hamburger, and a bottom bun. Note how the top bun and the bottom bun are very similar. The top bun, in a way, is like a topic sentence, and the bottom bun is like the concluding sentence. Both buns "hold" the meat, onions, and so on. Similarly, the topic sentence and concluding sentence "hold" the supporting sentences in the paragraph.
Not all academic paragraphs contain concluding sentences, especially if the paragraph is very short. However, if your paragraph is very long, it is a good idea to use a concluding sentence.
Details in Paragraphs
Whenever possible, you should include enough details in your paragraphs to help your reader understand exactly what you are writing about. Why are details important? Consider the example of the hamburger, mentioned above.* If the hamburger buns are the topic and concluding sentences, then the meat, the cheese, the lettuce, and so on are the supporting details. Without the food between the hamburger buns, your hamburger would not be very delicious! Similarly, without supporting details, your paragraph would not be very interesting.
A Note on Formality. In addition to having a particular kind of structure, academic paragraphs are different from "ordinary writing" in that certain kinds of expressions are not allowed. For example, in formal essays, you should not use contractions such as don't or aren't. Instead, you should write out the words in full, for example, do not and are not.
Also, in formal essays you should avoid the first and second person. That is, do not use the pronouns you or I. The pronouns we and us are sometimes used in formal essays in some major fields, but in general you should not use these unless you are certain that they are customary in your field and/or your professor allows them. It is safer simply to use the third person.