PARAGRAPH WRITING:

 If you look at any printed prose book, you will see that each chapter is divided up into sections, the first line of each being usually indented slightly to the right. These sections are called Paragraphs. Chapters, essays and other prose compositions are broken up into paragraphs, to make the reading of them easier, for the beginning of a new paragraph marks a change of topic, or a step in the development of an argument or of a story. In writing essays or other compositions, it is important to know how to divide them properly into paragraphs; for an essay not so broken up, looks uninteresting and is not easy to read.
Definition:- 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.
These definitions show that the paragraphs of a composition are not mere arbitrary divisions. The division of a chapter into paragraphs must be made according to the changes of ideas introduced.
There is, therefore, no rule as to the length of paragraphs. They may be short or long according to the necessity of the case. A paragraph may consist of a single sentence, or of many sentences.
(Note.-In. this respect, the paragraphs of a piece of prose differ from the stanzas or verses of a poem. The stanzas of a poem are usually of the same length and pattern; but paragraphs are long or short according to the amount of matter to be expressed under each head).
PRINCIPLES OF PARAGRAPH STRUCTURE 1. UNITY:- The first and most important principle to be observed in constructing a paragraph is that of Unity. Just as each sentence deals with one thought, each paragraph must deal with one topic or idea- and with no more than one. In writing an essay, for example, every head, and every sub-head, should have its own paragraph to itself. And every sentence in the paragraph must be closely connected with the main topic of the paragraph. The paragraph and every part of it must be the expression of one theme or topic.
(Note:- A good practice is to read a chapter in a book, and give a short heading or title to each paragraph, which will express in a word or brief phrase the subject of the paragraph).
   
The topic, theme or subject of a paragraph is very often expressed in one sentence of the paragraph - generally the first. This sentence is called the topical sentence (because it states the topic), or the key-sentence (because it unlocks or opens the subject to be dealt with in the paragraph).
2.ORDER:- The second principle of paragraph construction is Order - that is, 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 leading idea and arranged according to their importance or order.
(Note:- The two most important sentences in the paragraph are the first and the last. The first, which should as a rule be the topical sentence, should arouse the interest of the reader; and the last should satisfy it. The first, or topical, sentence states the topic - a fact, a statement, or a proposition; the last should bring the whole paragraph on this topic to a conclusion, or summing up).
3.VARIETY:- A third principle of paragraph construction is Variety; by which is meant that, to avoid monotony, the paragraph of composition should be of different lengths, and not always of the same sentence construction.
To sum up:- the essentials of good paragraph construction are - (1) Unity. (2) A good topical sentence. (3) Logical sequence of thought. (4) Variety. (5) A full and rounded final sentence in conclusion.
EXAMPLES Now let us examine a few paragraphs by standard authors, in illustration of these principles of paragraph construction.
1. “Hence it is that it is almost a definition of a gentleman to say he is one who never inflicts pain. This description is both refined and, as far as it goes, accurate. He is mainly occupied in merely removing the obstacles which hinder the free and unembarrassed action of those about him; and he concurs with their movements rather than takes the initiative himself. His benefits may be considered as a parallel to what are called comforts or conveniences in arrangements of a personal nature, like an easy chair or good fire, which do their part in dispelling cold and fatigue, though nature provides both means of rest and animal heat without them.”  - J.E. Newman.
This is a paragraph from Cardinal Newman's famous description of a “Gentleman” in his The Idea of a University, Notice that the paragraph is confined to one point in the character of a gentleman, which is clearly stated in the first, or topical sentence viz., that "he is one who never inflicts pain." The rest of the paragraph is simply a development and illustration of the topical sentence. And the concluding sentence drives home the statement of the subject with its similies of the easy chair and the good fire.
 2. "The Road is one of the great fundamental institutions of mankind. Not only is the Road one of the great human institutions because it is fundamental to social existence, bin also because its varied effects appear in every department of the State, It is the Road which determines the sites of many cities and the growth and nourishment of all. It is the Road which controls the development of strategies and fixes the sites of battles. It is the  Road that gives framework to all economic development. It is the Road which is the channel of all trade, and, what is more important, of all ideas, In its most humble function

it is a necessary guide without which progress from place to place would be a ceaseless experiment; it is a sustenance without which organised society would be impossible, thus the Road moves and controls all history." - Hilaire Belloc.
In this paragraph, the first sentence states the subject. It is the topical sentence. The body of the paragraph consists of examples which prove the statement in the first sentence. The final sentence sums up the whole.
3. "Poetry is the language of the imagination and the passions. It relates to whatever gives immediate pleasure or pain to the human mind. It comes home to the bosoms and businesses of men; for nothing but what comes home to them in the most general and intelligible shape can be a subject for poetry. Poetry is the universal language which the heart holds with nature and itself. He who has a contempt for poetry cannot have much respect for himself, or for anything else. Wherever there is a sense of beauty, or power, or harmony, as in the motion of a wave of the sea, in the growth of a flower, there is poetry in its birth." - William Hazjitt.
Here again, the first sentence is the topical sentence. The sentences that follow enforce or restate the statement that "poetry is the language of the imagination and the passions"; and the concluding sentence reinforces it by showing that poetry exists wherever men feel a sense of beauty, power or harmony.
In all these paragraphs, the principles of Unity and Order are observed, and also the general rules about the place of the topical sentences and the rounding off the whole with a good conclusion.
THE WRITING OF SINGLE PARAGRAPHS So far we have been treating of paragraphs which are sections of a more or less lengthy composition, like an essay or the chapter of a book. But students are often asked in examinations to write short separate paragraphs, instead of essays, on subjects of ordinary interest. Such single paragraphs are really miniature essays; but the same principles as we have discussed above (except the principle of variety), must be followed in their construction. Each paragraph must be a unity, treating of one definite subject, and must follow a logical order of thought. In most cases, too, the rules about the topical sentences and the conclusion should be borne in mind.
A few examples should make this clear. Suppose, for example, you are asked to write a paragraph on “The Cat.” It is obvious that you cannot treat this subject fully, as you might in a long essay. And yet you must, according to the principle of unity, confine your paragraph to one definite topic. You must, therefore, choose one thing to say about a cat, and stick to it throughout. You might, for example, write of one characteristic of the cat, say, its love of comfort and attachment to its home. In that case, you might write a paragraph something like this:-

The Cat There is some truth in the common saying that while dogs become attached to persons, cats are generally attached to places. A dog will follow his master anywhere, but a cat keeps to the house it is used to; and even when the house changes hand, the cat will remain there, so long as it is kindly treated by the new owners. A cat does not seem to be capable of the personal devotion often shown by a dog. It thinks most of its own comfort and its love is only cupboard love.  

Notice the construction of this paragraph. It begins with the topical sentence, which clearly states the subject. The following sentence explains the statement by expanding it; and the last sentence, by giving a reason for the attachment of a cat to a particular house forms a fitting conclusion. The paragraph is therefore a Unity, treating of one characteristic of cat character: and it follows an orderly plan.
The paragraph on the cat is descriptive. Now take an example of a narrative paragraph, in which you are required to tell a story. Suppose the subject is to be a motor-car accident; you might treat it in this way:-
A Motor-Car Accident It is the mad craze for speed that is responsible for many motor accidents. Only last year I witnessed what might have been a fatal accident on the Kashmir Road. I was motoring down from Srinagar; and as I was nearing Kohala, I came upon the wreckage of two cars on the road. The smash had been caused by a car coming down, which swept round a sharp comer at forty mites an hour and crashed into a car coming up. Happily no one was killed; but several were badly injured, and the two cars were wrecked. To drive at such a speed down a twisting mountain road is simply to court disaster.
In this paragraph, the topical sentence is again first; the narrative that follows is simply an illustration of the statement in the topical sentence that many accidents are caused by a mad craze for speed; and the concluding sentence sums the paragraph up by a restatement of the topical sentence in other words.
The following is an example of a reflective paragraph; that is, one that expresses some reflection or thought on an abstract subject:-
Mercy To forgive an injury is often considered to be a sign of weakness; it is really a sign of strength. It is easy to allow oneself to be carried away by resentment and hate into an act of vengeance; but it takes a strong character to restrain those natural passions. The man who forgives an injury proves himself to be the superior of the man who wronged him, and puts the wrong-doer to shame. Forgiveness may even turn a foe into a friend. So mercy is the noblest form of revenge.
The topical sentence of a paragraph is usually the first, or at latest the second; and this is the best place for it. But for the sake of variety it may be placed in a different position. In this paragraph, it comes last - "So mercy is the noblest form of revenge". But the opening sentence is also a good introduction to the subject, and is calculated to arouse interest by stating an apparent paradox.
To sum up:- In writing single paragraphs, the principles of Unity and Order must be kept in mind, and also the rules of the topical and concluding sentences. The language should
be simple, the style direct, and the sentences short; and, as a paragraph is limited, all diffuseness must be avoided.
Exercise 132.  Write short paragraphs on the following subjects:- 1. A Rainy Day.  2. A Walk.  3. The Cow.  4. Trees.  5. Politeness.  6. Anger.  7. A picnic.  8. A Fire.  9. A Flood.  10. Some Pet Animal.  11. Rivers.  12. Cricket.  13. Contentment.  14. Gymnastics.  15. Gratitude.  16. A Holiday.  17. The Elephant.  18. The Cobra.  19. The Tailor.  20. The Astronaut.  21. Revenge.  22. Thrift.  23. Stars.  24. The Crow   25. Robots  26. To-day’s Weather.  27. Your hobby.  28. Humility.  29. The Mango.  30. Examination.

( SOURCE : GRAMMAR BOOK BY WREN AND MARTIN)