Paragraph 1. Sentence 1 is the topic sentence, because Sentences 2, 3, 4, 5, 6 and 7 aredetailed explanation, while Sentence 8 is a further summary and conclusion of the topic. In suchcases, the main idea is both at the beginning and the end. But the reader must be clear that the lastsentence is much more accurate in summarizing the main idea than the first sentence. Therefore, themain idea of the paragraph should be that the way of behaving politely differs from country to country.
Paragraph 2. The topic sentence is not the first sentence, but the second, because the othersentences including sentences 3, 4 and 5 are the supporting details, further explaining and provingthe second sentence. Therefore, the main idea is closely related to the second sentence, which can besummarized as. The physical assets of the cockroach provide a major reason for its survival.
Paragraph 3. There is no stated topic sentence, but all the sentences describe steps of sending aparcel. So the main idea is how I sent a parcel by post.
Paragraph 4. The last sentence is the topic sentence, because the other sentences are examplesto illustrate Americans' attitude towards crying.