Atopic sentence, capable of development, must focus like a spotlight on three characteristics. what are these characteristics?

A topic sentence (or focus statement) is written with the purpose of telling your reader the main point of your writing. Topic sentences can summarize, suggest a thought, or introduce a topic. 

Paragraphs are short, so topic sentences need to be specific. It is clear and easy to follow. You can't cover a large amount of information in a short space, so making the topic sentence narrow will provide a specific point to be supported. 

A topic sentence should support the rest of the paragraph. Often references such as quotations, references from documents, or statistics are used as evidence to support an idea. 

No matter the form of writing you are taking on, there will be a question or claim that you are trying to tackle. Your topic sentence should provide answers to address the point you are trying to make.