First, there are obvious signs, such as usage theory, discussion, discussion, etc.
Second, the topic of the paper is often a topic, straight and straight, such as life, Bing Xin, and Zhu Ziqing's prose art.
Third, there can be two topics, besides the main topic, there can also be a sub-topic (or sub-topic), such as China's masterpiece of ancient novels-on a dream of red mansions and so on.