This main idea of each paragraph is usually expressed somewhere in the paragraph by one sentence (the main or topic sentence). This sentence is usually found at the beginning of the paragraph, but can come at the end or even in the middle of the paragraph. The rest of the paragraph generally expands the theme contained in the main sentence, and each idea round the main theme is supported by information and evidence (in the form of illustrations and examples), and by argument.
The population as a whole was unevenly distributed. The north was particularly thinly settled and the east densely populated, but even in counties like Warwickshire where there were substantial populations, some woodland areas were sparsely peopled. There was already relatively dense settlement in the prime arable areas of the country like Norfolk, Suffolk and Leicestershire. Modern estimates of England's total population, extrapolated from Domesday patterns, vary between 1 and 3 million.
(Asa Briggs, (1983). A social history of England, p. 58)
Atoms of all elements consist of a central nucleus surrounded by a "cloud" containing one or more electrons. The electrons can be thought of as occupying a series of well-defined shells. The behaviour of a particular element depends largely on the number of electrons in its outermost shells. Other factors, such as the total number of electron shells, also play a part in determining behaviour but it is the dominance of the outer electron configuration that underlies the periodic law and justifies the grouping of the elements into groups or families.
(The sciences: Michael Beazley Encyclopaedias (1980), p. 118)
In general, Victorian families were big. In 1851 their average size was 4.7, roughly the same as it had been in the seventeenth century, but the 1½ million couples who married during the 1860s, which the historian G. M. Young described as the best decade in English history to have been brought up in, raised the figure to 6.2. Only one out of eight families had one or two children, while one in six had ten or more, so that the counsel 'little children should be seen and not heard' was prudent rather than simply authoritarian advice.
(Asa Briggs, (1983). A social history of England, p. 244)
The spoken word (whether conversation or oratory or the coy mixture of the two which is now familiar to us on television) is a very different thing from the written word. What is effective or allowable or desirable in the one may be quite the reverse in the other, and the extempore speaker cannot correct himself by revision as the writer can and should. It is therefore not fair to take a report of a speech or of an oral statement and criticise it as if it were a piece of considered writing.
(Ernest Gowers, (1973) The complete plain words, p. 26)
Identifying topic sentences
Look at the structure of the following paragraph.
This is a period when education faces many disturbing circumstances originating outside itself. Budgets have been drastically cut throughout the country affecting every type of education. Enrolments are dropping rapidly, because the children of the post-World War II "baby boom" have now completed their schooling, and we are feeling the full effect of the falling birth rate. So there are fewer opportunities for new teachers, and the average age of teachers is increasing.
(Carl Rogers, (1969), Freedom to learn, p. 11.)