The topic sentence is a statement of the subject, or topic, of the paragraph, often found at the beginning of the paragraph. It may introduce an impression to be developed in specific details, or a point to be examined, or a question to be answered. .Sometimes the topic sentence is a complete statement of the main, or topic idea. Besides, the topic idea may be repeated or restated at the end, following the details of the discussion.

For example:

Hong Kong is faced with the difficult problem of illegal immigrants. Thousands of people enter Hong Kong illegally every year. They hope to make a lot of money. However, they don't realize that they have to work very hard to earn their money and that the cost of living is very high. They expect life to be easy. In fact, they find that it is very tough. As a result, some of them turn to crime. Unfortunately many people try to get here that the police has a hard time to keep them out. It is a difficult problem but we must do what we can to solve it.

`Hong Kong is faced with the difficult problem of illegal immigrants.' is the topic sentence. The second sentence explains what the illegal immigrants are and what they aim at.

Then, it is followed by what they have mistaken. Finally, it comes to the consequence of the mistake and why it costs a difficult problem to Hong Kong.

Occasionally the topic sentence appears toward the middle of the paragraph, or even at the end, proceeded by a series of details that build to it. The topic sentence is said to be implied when the details make the point without an explicit statement of the topic idea.

Main and Subordinate ideas

In longer paragraphs, the main idea may be distinguished by repeating or restating it in the conclusion. When one organizes the paragraph, one has to keep in mind that the beginning and ending are usually the most forceful parts because of their importance.