A topic sentence is the most important sentence in a paragraph.
Sometimes referred to as a focus sentence, the topic sentence helps
organize the paragraph by summarizing the information in the paragraph.
In formal writing, the topic sentence is usually the first sentence in a
paragraph (although it doesn't have to be).
Purpose of the Topic Sentence
A topic sentence essentially tells readers what the rest of the paragraph is
about. All sentences after it have to give more information about that
sentence, prove it by offering facts about it, or describe it in more detail.
For example, if the topic sentence concerns the types of endangered
species that live in the ocean, then every sentence after that needs to
expound on that subject.
Topic sentences also need to relate back to the thesis of the essay. The
thesis statement is like a road map that will tell the reader or listener where
you are going with this information or how you are treating it.
Topic Sentences and Controlling Ideas
Every topic sentence will have a topic and a controlling idea. The
controlling idea shows the direction the paragraph will take.
Here are some examples:
Topic Sentence: There are many reasons why pollution in ABC Town is
the worst in the world.
The topic is “pollution in ABC Town is the worst in the world” and the
controlling idea is “many reasons.”
A "clincher" sentence is one which offers a summary and conclusion that is
logically unassailable and delivered in powerful language. It is often
preceded by several sentences which pull together the threads of evidence
or argument, and then delivers the knock-out punch as the "clincher."