There must be only one topic idea which, when expressed in only one sentence, is called the topic sentence. Though the topic idea is a must for paragraphs, the topic sentence is not. That is, there may be a stated or an implied topic sentence.
In most descriptive paragraphs, it's implied. The description itself shows what's being described. Again, narratives may or may not have such topic sentences. But in most of the expository paragraphs (or essays) written by students, there are clearly stated topic sentences. Such a topic sentence, if any, may be placed in the beginning or ending or anywhere. However, in paragraphs (or essays) written in the 'particular to general' order, it appears in the ending, while in those written in the 'general to particular' order, it appears in the beginning. The latter order being followed by most students in their academic reports, the topic sentence appears in the beginning in such cases. Both orders, however, tend to be equally frequently used in articles written for magazines.
Not that in expository paragraphs the topic sentence can not be kept implied. It can. Here's an example.
Where does a baby look the nicest? In its mother's lap. And where do the animals look the most beautiful? Surely in the jungle. We may encage a bird out of joy and with a view to enjoying its beauty and song, but after some days it will lose all its attractiveness. Then the bird flying at large up in the sky will look better than that encaged one. People are seldom presented with wild flowers by their beloveds. Why? Perhaps they do not consider wild flowers beautiful when they are brought home. But it is surprising to see how eagerly people look at them in the wilderness. This is because wild flowers are beautiful in where they bloom. A beautiful woman will not look beautiful in the dirty boisterous factory, but in the flower garden she will. Similarly, a young stout-bodied man will not look nice when he is working in the kitchen. What does it all imply, then?
This paragraph has no topic sentence, but it can be inferred that the topic idea is : Everything has its own place. And this would be the topic sentence if it were stated in the paragraph. Truly speaking, in this case, the paragraph would be more beautiful and easier if this topic sentence were added to it at the ending. Here's another example.
I remember once meeting a man who was bundling up his accessories in a hotel room. He was trying to work so swiftly that he seemed to be in a hurry. When asked why he was being so busy, he said that he must catch the train which would leave after one hour. "Why so hurry? You've ample of time." I assured him. But he would not listen to me. Then I told him. "O.K. make haste slowly." Yet he was being busier and busier. While bundling his articles, he broke some glass wares. As soon as he started moving. he remembered his towel and brush being left out. Then he took them but could not keep them in the briefcase because he could not remember where he had kept the key. Then, after much searching and rummaging, he found the key under the pillow. Then again he was about to set off when he reached for the train ticket, but it was not there. "My ticket! My ticket!" he muttered to himself worriedly. At last he found it in his purse. Then he set off.
After half an hour he came back, panting and sweating from head to foot. "What's wrong again?" I asked him. He said that he had missed the train. Had he not made haste, he would not have missed the train. Now he suffered much a waste of time and money.
The topic idea of this paragraph can be easily understood from the penultimate sentence (the sentence prior to/before the last). The topic sentence would be: Haste makes waste.
That there can be a stated topic sentence in a descriptive paragraph can also be exemplified. The topic sentence of the following paragraph is implied.
When the tired sun is about to skip the wall of the horizon at the end of the day, it leaves the last touch of its warmth and blush scattered in the west sky. The tiny waves of the river water playfully reflect the fading rays like diamond. Nature pauses for a while to watch the beauty of the universe, the last smile of the day. Everywhere is sorrounded by the calm of rest and leisure. Tree tops glow feverishly; patches of lonely cloud wink. From the south blows soft pacifying gentle breeze. The sun, blinking several times looking at the earth, says goodbye to her. Down comes the evening, but the remnant of the fluorescent twilight glares in the mind's eye of the beholder.
But the following sentence could be added to it at the very beginning keeping the unity of the paragraph intact:
A sunset scene, though not an unusual phenomenon in our everyday experience, can become a unique source of enjoyment to a humorist heart.
In that case this sentence would be the topic sentence. Read the above paragraph again placing the above sentence at the beginning, and you'll understand it.
In fact, most of the topic ideas of descriptive and narrative paragraphs are plain facts or perceptions; they need little special effort to be understood. That's why a discussion of the topic sentence in that regard is not very important. On the other hand, because expository paragraphs make the reader think and argue with himself or herself, topic sentences are an important factor to them. Consequently, you have to think in advance about whether any topic sentence is to be stated in the paragraph at all, and about where it should be placed, if used. In deciding about this point, a selection of a suitable order in advance should be very helpful.
