In formal writing, the topic sentence is usually the first sentence in a paragraph (although it doesn't have to be).
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.
The topic part names the topic; it tells what the paragraph is about.
The controlling idea part tells what the paragraph will say about the topic. Usually, the topic comes first and the controlling idea comes second in the topic sentence.
