A topic sentence acts as the controlling or directing sentence of a paragraph. Key terms deployed in a topic sentence determine what your reader expects from the rest of the paragraph and so governs what you must provide them with. One way of understanding a topic sentence is to see it as a claim that you must then justify as you complete the paragraph.

Let's say that you are writing about the opening of Jouney's End, here's a topic sentence that you might open a paragraph with:

Sherrif immediately involves the audience in the action of the play through the use of seemingly incongruous humour.

This sentence provides your reader with the key concepts of humour, incongruity (when something is out of place), and audience involvement. The rest of the paragraph needs to develop the ideas mentioned here. In this sense a topic sentence is rather like a mini thesis statement.

Following an opening like this you might want to elaborate on the nature of the humour that is a feature of the opening of the play, using quotation to support your point, and describe the way in which this humour determines the relationship between the audience and the characters. You may also comment on the way in which this humour seems at odds with the sombre setting of the trench and speculate as to the reasons for this seeming incongruity.

One way of concluding a paragraph is to refer back to the topic sentence (just as a good essay conclusion will refer back to the introduction) and sum up the main point/claim that has been asserted. For example, a concluding sentence for our model topic sentence and paragraph might be something like this:

Thus Sherrif engages the audience at the beginning of the play by suggesting that even in wartime the human desire to amuse each other and make light of life's difficulties persists.