If you write a long paragraph (more than 4 or 5 sentences), how do you keep focused on the topic? How do you keep the reader aware of the main idea being discussed?
You do this with topic chains. A topic chain is basically a series of words and phrases that refer to the main idea. In most cases when you use a topic chain, each sentence will have one or more words that refer to the idea. If this is not possible with a particular sentence, you may need to consider whether or not that sentence belongs in the paragraph. Consider this paragraph from a proposal for state authorization to provide after school services to at-risk children.
The term disabilities comprises many conditions that may inhibit student learning. Often, students with disabilities require specialized instructional strategies to reduce the degree to which these inhibitors affect learning. Students with special needs require a highly-qualified teacher with training and experience in addressing such needs. As part of the tutor selection process, [the company] identifies those teachers possessing these unique skills, resulting in the ability to match students with special needs with teachers possessing appropriate teaching skills. Teachers will use strategies that allow for differentiated pacing with careful sequencing, monitoring, and control of the learning process.
The underlined words create the topic chain. As you can see, each sentence contains words that refer to the topic introduced in the first sentence. These words keep the reader focused on the topic.