In written form, English is divided into Paragraphs to distinguish one main idea from other main ideas. The paragraph is the basic unit of composition (=essay, article).
1- A Paragraph is a set of sentences which develop one central idea.
2- The central idea is usually expressed in a topic sentence.
3- Every sentence in the paragraph must help the development of the topic sentence.
The first sentence of a paragraph is always indented so that the reader will know that a new subject—or a different aspect of the same subject—is being dealt with. The writer does this by leaving a blank space at the beginning of the paragraph. Think of indentation as simply another kind of Punctuation. Just as a sentence ends with a period (.), so each new paragraph begins with an indentation.
A paragraph may vary in length. Some paragraphs are quite short; others are extremely long. Most paragraphs have more than three sentences in them and usually have between 100 and 200 words.
Topic VERSUS Topic Sentence
The first sentence of a paragraph is usually called the topic sentence. You may have noticed in your reading of certain English texts that it is possible to place the topic sentence at the end of the paragraph (as a kind of conclusion), or even in the middle (as a kind of link between the two parts). It is even possible not to have a topic sentence at all; in this case, we say that the topic sentence is implied or suggested. Be aware of these possibilities when you read; otherwise, you might miss the point which the author is trying to make. When you write, however, remember that a topic sentence placed at the beginning of a paragraph is the clearest kind of paragraph organization— simple, effective, easy for you to manage, and easy for your reader to understand.
Topic Sentence = TOPIC + LIMITING STATEMENT
Take, for example, the general topic of soccer. There are too many things to say about soccer to put into a single paragraph. Therefore, your problem as a writer consists of deciding how you want to write about soccer. In other words, you need to limit your discussion.
One good way to limit your topic is to place key words or phrases in the topic sentence. These words or phrases will let the reader know how you are going to discuss the topic. These words or phrases are sometimes called controlling words or Phrases, since they control the organization of the paragraph. In a paragraph on soccer, for example, they will immediately indicate to the reader that you plan to do one of several things:
Discuss the history of soccer
Compare it with another sport
Describe its difficulty
Explain the rules of the game
How do you limit a topic in a topic sentence?
There are many ways, but the following is a list of the most common. Once you understand these examples, you will find it much easier to write a carefully controlled topic sentence.