The Topic Sentence
 

(Notice how the first sentence begins with "My hometown..." a few spaces to the right of the paragraph edge.  This is an indentation.  All paragraphs in English MUST begin with an indentation.)

Note how the first sentence, My hometown, Wheaton, is famous for several amazing geographical features,is the most general statement. This sentence is different from the two sentences that follow it, since the second and third sentences mention specific details about the town's geography, and are not general statements.

Here are some examples of sentences that cannot be used as topic sentences. Can you figure out why they are inappropriate?
 

The problem with sentence #1 is that it contains too many details. Topic sentences are general, and details should appear later in the paragraph. A better topic sentence would be like the one mentioned above, My hometown is famous for several amazing geographical features.

Sentence #2 is not appropriate as a topic sentence because it mentions two topics, not just one. Paragraphs are usually about one main thing and so their topic sentences should also be about only one main thing.

The problem with sentence #3 is that it is toogeneral. It is also very boring! Would you like to read a paragraph with this topic sentence? Most people would not.

We can rewrite sentences #2 and #3 in the following ways to make it better:
 

Supporting Sentences
 

Consider again the above-mentioned, short paragraph:
 

(Again, note how this paragraph is indented on the first line, about five or seven spaces in from the left-hand edge of the paragraph. Alwaysremember to indent your paragraphs!)

When a reader reads a topic sentence, such as My hometown, Wheaton, is famous for several amazing natural features,a question should usually appear in the reader's mind.  In this case, the question should be like, "What are the natural features that make Wheaton famous?"   The reader should then expect that the rest of the paragraph will give an answer to this question. 

Now look at the sentences after the topic sentence.  We can see that the second sentence in the paragraph,  First, it is noted for the Wheaton River, which is very wide and beautiful,indeed gives an answer to this question. That is, the second sentence gives some explanation for the fact that Wheaton is a famous town. Similarly, we can see that the third sentence also gives some explanation for the fact that Wheaton is famous by giving another example of an "amazing natural feature," in this case, Wheaton Hill.

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.   The paragraph above is actually a very short paragraph.  At minimum, you should have at least five to seven sentences in your paragraph.  Here we can see our paragraph about Wheaton with a few more supporting sentences in bold font:
 

In this lesson, we will talk about supporting sentences again in the section, "Details in Paragraphs," below.
 

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.  Let's see how a concluding sentence (in bold font) might look in our sample paragraph about Wheaton:
 

Notice how the concluding sentence, These three landmarks are truly amazing and make my hometown a famous place,summarizes the information in the paragraph.  Notice also how the concluding sentence is similar to, but not exactly the same as, the topic sentence. 

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
 

The short paragraph in this lesson is a fairly complete paragraph, but it lacks details.  Whenever possible, you should include enough details in your paragraphs to help your reader understand exactly what you are writing about.  In the paragraph about Wheaton, three natural landmarks are mentioned, but we do not know very much about them.  For example, we could add a sentence or two about Wheaton river concerning HOW wide it is or WHY it is beautiful.  Consider this revision (and note the additional details in bold):
 

If we wished, we could also add more details to the paragraph to describe the third natural feature of the area, the Big Old Tree.

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 (and multi-paragraph essays, which will be topic of another lesson) are different from "ordinary writing" (such as letter 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 I or you.  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.

Study the following table to learn about some informal expresions that you should avoid in academic writing:
 

Source: http://lrs.ed.uiuc.edu