Ordinarily, however, a subject requires subdivision into topics, each of which should be made the subject of a paragraph. The object of treating each topic in a paragraph by itself is, of course, to aid the reader. The beginning of each paragraph is a signal to him that a new step in the development of the subject has been reached. (来源：英语学习门户 http://www.EnglishCN.com)
The extent of subdivision will vary with the length of the composition. For example, a short notice of a book or poem might consist of a single paragraph. One slightly longer might consist of two paragraphs:
A. Account of the work.
B. Critical discussion.
A report on a poem, written for a class in literature, might consist of seven paragraphs:
C. Facts of composition and publication.
D. Kind of poem; metrical form.
F. Treatment of subject.
G. For what chiefly remarkable.
H. Wherein characteristic of the writer.
I. Relationship to other works.
The contents of paragraphs C and D would vary with the poem. Usually, paragraph C would indicate the actual or imagined circumstances of the poem (the situation), if these call for explanation, and would then state the subject and outline its development. If the poem is a narrative in the third person throughout, paragraph C need contain no more than a concise summary of the action. Paragraph D would indicate the leading ideas and show how they are made prominent, or would indicate what points in the narrative are chiefly emphasized.
A novel might be discussed under the heads:
A historical event might be discussed under the heads:
N. What led up to the event.
O. Account of the event.
P. What the event led up to.
In treating either of these last two subjects, the writer would probably find it necessary to subdivide one or more of the topics here given.
As a rule, single sentences should not be written or printed as paragraphs. An exception may be made of sentences of transition, indicating the relation between the parts of an exposition or argument.
In dialogue, each speech, even if only a single word, is a paragraph by itself; that is, a new paragraph begins with each change of speaker. The application of this rule, when dialogue and narrative are combined, is best learned from examples in well-printed works of fiction.
2. As a rule, begin each paragraph with a topic sentence; end it in conformity with the beginning.
Again, the object is to aid the reader. The practice here recommended enables him to discover the purpose of each paragraph as he begins to read it, and to retain the purpose in mind as he ends it. For this reason, the most generally useful kind of paragraph, particularly in exposition and argument, is that in which
A. the topic sentence comes at or near the beginning;
B. the succeeding sentences explain or establish or develop the statement made in the topic sentence; and
C. the final sentence either emphasizes the thought of the topic sentence or states some important consequence.
Ending with a digression, or with an unimportant detail, is particularly to be avoided.
If the paragraph forms part of a larger composition, its relation to what precedes, or its function as a part of the whole, may need to be expressed. This can sometimes be done by a mere word or phrase (again; therefore; for the same reason) in the topic sentence. Sometimes, however, it is expedient to precede the topic sentence by one or more sentences of introduction or transition. If more than one such sentence is required, it is generally better to set apart the transitional sentences as a separate paragraph.
According to the writer's purpose, he may, as indicated above, relate the body of the paragraph to the topic sentence in one or more of several different ways. He may make the meaning of the topic sentence clearer by restating it in other forms, by defining its terms, by denying the converse, by giving illustrations or specific instances; he may establish it by proofs; or he may develop it by showing its implications and consequences. In a long paragraph, he may carry out several of these processes.
1 Now, to be properly enjoyed, a walking tour should be gone upon alone. 1 Topic sentence.
2 If you go in a company, or even in pairs, it is no longer a walking tour in anything but name; it is something else and more in the nature of a picnic. 2 The meaning made clearer by denial of the contrary.
3 A walking tour should be gone upon alone, because freedom is of the essence; because you should be able to stop and go on, and follow this way or that, as the freak takes you; and because you must have your own pace, and neither trot alongside a champion walker, nor mince in time with a girl. 3 The topic sentence repeated, in abridged form, and supported by three reasons; the meaning of the third ("you must have your own pace") made clearer by denying the converse.