Identify the part of speech of the word "is" in the sentence "A dog is a very good pet."

The word "is" in the sentence "A dog is a very good pet" serves as a verb. Specifically, it is a form of the verb "to be," which is used to indicate existence or a state of being. In this context, "is" connects the subject "a dog" with the subject complement "a very good pet," effectively stating what the subject is. Verbs are essential in sentences as they express actions, occurrences, or states of being, and in this case, "is" functions to express the state of the dog as being a good pet.