ESLBEE.COM contains a few examples of topic sentence on this page: Topic Sentence Exercises.
It was stated earlier that a topic sentence includes a topic and a controlling idea. Looking at Number One on the left, the topic (in red) is avoiding burglaries. The controlling idea, then (in green) is taking certain precautions. What precautions can someone take to avoid burglaries? Installing an alarm system, buying a guard dog, and installing stronger locks are three precautions one could take. Students in the past have given at least a dozen ways to avoid burglaries. By thinking through your own topic and controlling statement before starting to write, the writing itself becomes extremely easy because you already know what you are going to write.
Another interesting example is Number Ten. The topic is exploring the wreck of the Titanic. What are the problems scientists must overcome to explore the wreck? The depth of the water, the darkness, getting bright lights that can function at that depth, the pressure, the time it takes to descend and ascend. Again, once the topic is chosen, the writer has to decide what he/she is going to discuss about the topic, so a well-chosen (often simple) "controlling idea" is crucial.
Before writing any essay or even a paragraph, it is important to think, first about the topic and then what you want to say about the topic. Most often, the topic is easy, but the question then turns to what you want to say about the topic which is the controlling idea. Topic sentences should always contain both (1) a topic and (2) a controlling idea.
Read the following topic sentences. They all contain a topic (in red) and a controlling idea (in green). When your introduction contains a clearly stated topic sentence such as one of the following, your reader will know what to expect and, therefore, understand your ideas better.
1. People can avoid burglaries by taking certain precautions.
(The precautions for…)
2. There are several advantages to growing up in a small town.
(The advantages of…)
3. Most US universities require a 550 point TOEFL score for a number of reasons.
(The reasons for…)
4. Air pollution in Mexico City is the worst in the world for a number of reasons.
(The causes of…) or (The effects of…)
5. Fixing a flat tire on a bicycle is easy if you follow these steps.
(The steps for…)
6. There are several enjoyable ways to travel between the US and Queretaro.
(The ways to…) or (The methods of…)
7. Animals in danger of becoming extinct come from a wide range of countries.
(The different countries… [parts, kinds, types])
8. Effective leadership requires specific qualities that anyone can develop.
(The qualities (or characteristics or traits) of…)
9. Industrial waste poured into Lake Michigan has led to dramatic changes in its ability to support marine life.
(The effects of…)
10. In order to fully explore the wreck of the Titanic, scientists must address several problems.
(The difficulties of…) or (The obstacles to…)
A carefully thought out topic sentence will serve two important functions. First, it will provide you, the author, the means to stay focused on your objective. It's a lot easier to write if you know what you are going to write about! Second, a clearly stated topic sentence will provide readers with the tools they need to clearly understand what you have to say. These are important reasons to carefully consider topic sentences before putting pen to paper.