Writing an effective topic sentence
A good topic sentence introduces the topic to be developed in a paragraph. It controls or limits the focus of paragraph, and once the focus is mentioned, there is no scope to go outside it. Anything outside its scope will be redundant or unnecessary. On the other hand, if the paragraph does not sufficiently support the topic statement that proves another weakness of a paragraph.
Remember that although the topic sentence usually comes at the beginning of a paragraph, it may sometimes come at the end or even in the middle of the paragraph. Sometimes the main idea of a paragraph can be so obvious that it need not be stated explicitly.