用于介绍的“叫”字句和“是”字句

Jiao（叫）-sentences and shi（是）-sentences can both be used for introduction, but they differ in pragmatic function. Jiao-sentences are for initial introduction, bringing in the name of an entirely new entity into the cognition of the addressee, while shi-sentences are for supplementary introduction, adding information to the entities already known to the addressee. As a result, the two kinds of sentences can be used in different situations and therefore have different effects of expression. Their different pragmatic functions result from the differences between the object of jiao and shi: the former is a kind of metalanguage and the latter usually refers to the external world. However, influenced by the Politeness Principle, the hierarchy of salience and some language internal factors, jiao-sentences can sometimes be used for introducing the entities already known to the addressee, and shi-sentences can be used for introducing some unknown entities.