Sentences are a basic structure of language. They convey the action or existence of a person, place, or thing. Sentences are combined to form paragraphs to form longer written documents. This may sound simplistic, but to build an effective written communication, sentences have to be combined in certain ways to form the paragraphs which in turn an be combined to write longer works. Even the longest novel is made up of sentences which are organized into paragraphs except for dialogue. An essay is a special type of writing is a paper focused on proving a point called the thesis. Essays are composed of special types of paragraphs with very particular content.
The rules for punctuation and sentence structure are covered in the Grammar section. This section will cover how to compose paragraphs and an academic essay which is also, generally, the way beginning level research papers are organized. Research papers are also called research essays.