“该”类助动词在与其他类助动词连用时所处位置相同,而且在语义上都可以表达必要和必然两类模态。而这两种模态在形式上是有区别的,表现在能否受程度副词修饰、能否用“不”否定、正反问形式、能否与“了_1”共现、能否进入从句、能否带从句宾语等,据此可以预测出现在不同环境中的“该”类助动词所表达的意义。必要和必然虽分别属于道义模态和认识模态,但二者具有相同的逻辑本质,即必然性。这种联系在很多语言中都有表现。 “ ” And the two modalities are different in form, which can be modified by degree adverbs, can we use the “no” negative, positive and negative forms, can we co-exist with “_1” Whether or not to enter the clause, can take the clause object, etc., which can predict the meaning of the “the ” class auxiliary verbs that appear in different environments. Necessity and necessity, though belonging to moral modality and cognitive modality respectively, have the same logical essence, ie, necessity. This connection has many languages.