When you write something, (specifically when you write a paragraph), you have to write a topic sentence and you have to write a lot of supports for this.
The following exercises were made for practice the detection of the paragraphs's topic sentences.
I.-In the following paragraphs, identify the topic sentence and the
key words in the topic sentence that the other sentences support.
1. John Bookmeyer is a versatile, well-rounded boy. In
addition to singing in his church choir, he is a drummer in a
small combo. He maintains a high average in his subjects at
school and still manages to play football and run on the track
team. Although he is very popular with other young people, he
doesn't let his social life interfere with his weekend job at the
drugstore.
In this situation we have that "John Bookmeyer is a versatile, well-rounded boy", and this is the topic sentence because makes an a kind of summary that have support by another description of the key words.
2. The first thing I noticed about the school was the litter in the schoolyard. Trash and paper were scattered everywhere. The building, which could have been very attractive despite its age, was dingy and dirty beyond belief. A window -pane was broken out of the main door, and the door itself led into a dark, for bidding hall. As I walked down the hall, I noticed the messy bulletin board and, in several places, writing on the wall. No one greeted me when I entered the outer office, although a receptionist was sitting there noisily chewing a huge wad of gum and putting on fingernail polish. All in all. I couldn't help feeling that the school was poorly cared for.
In this case we have that the last phrase is the topic sentence. After all the phrases about description of the school, we can see the thought of the writer and the point of the paragraph.
3. Through the fog the arc lights high above the stadium looked like small moons from the field below. The lights were reflected all over the marshy field in pools of water, an accumulation of twenty four hours of steady drizzle. Despite lengthy warm-up drills, the players now felt the cold and dampness beginning to penetrate. They were either huddled together or dancing up and down to keep warm-- that is, all except our first-string quarterback and our all-state offensive end, both of whom were out of the game with injuries. They sat glumly on the bench wrapped in soggy woolen blankets, Conditions could hardly have been worse for this crucial game.
Once again we have the topic sentence at the end of the paragraph.
If you think, the topic sentences is like a summary. With a general idea and this idea is supported by a lot of points.
Have a topic sentence is useful for writing, because it allows that you can write a text with coherence and cohesion.