"A paragraph is a number of sentences grouped together and relating to one topic; or, a group of related sentences that develop a single point."
In writing essays or other compositions, it is important to know how to divide them properly into paragraphs. These paragraphs are not mere arbitrary divisions. The beginning of a new paragraph marks a change of topic, or a step in the development of a story or of an argument.
There is no rule as to the length of paragraphs. A paragraph may contain a single sentence or many sentences.
Principles of Paragraph Structure
UNITY – The most important principle to be observed in constructing a paragraph is that of Unity. Each paragraph must deal with one topic or idea. And, every sentence in the paragraph must be closely connected with the main topic of the paragraph. The theme or subject of a paragraph is often expressed in the first sentence of the paragraph. This sentence is called the key-sentence.
ORDER – The second principle of a paragraph construction is Order – i.e., the logical sequence of thought or development of the subject. Events must be related in the order of their occurrence, and all ideas should be connected with the main idea and arranged according to their importance and order.
The two most important sentences in the paragraph are the first and the last. The first sentence should evoke the interest of the reader, while the last sentence should bring the whole paragraph on this topic to a conclusion.
VARIETY - To avoid monotony, the paragraph of a composition should be of different lengths, and it should not always be of the same sentence construction.