Sentences with 是

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Subject              Predicate
                   adv   v是       N/NP           Pt
他                        是          老师
她                不     是          老师
小马                     是          学生            吗

In an  A 是B sentece,the verb 是 is  used to connect the two parts.Its  negative form is made by putting 不 before the verb  是，If the sentence is not particularly emphatic,是 is read softly.
Note ;  The adverb 不 must be placed before 是。