Which verb form is specifically mentioned as being a primary linking verb?

The verb form "to be" is recognized as a primary linking verb because it serves to connect the subject of a sentence to a subject complement, which can be a noun, pronoun, or adjective that describes or identifies the subject. This function is essential in constructing sentences that clarify or provide more information about the subject.

For example, in the sentence "She is a doctor," the verb "is" (a form of "to be") links the subject "She" to the complement "a doctor," establishing identity. Similarly, in "They are happy," the verb "are" connects the subject "They" to the adjective "happy," conveying a state of being.

Other verbs listed, like "to have," "to do," and "to know," serve different grammatical functions and don’t primarily function as linking verbs. "To have" indicates possession, "to do" relates to actions or tasks, and "to know" pertains to awareness or understanding. Thus, "to be" stands out as the primary verb specifically used for linking in English grammar.