Linking verbs connect a subject to a subject complement, which provides more information about the subject. They typically express a state of being, a sense of becoming, or a connection between the subject and the complement.
Examples of linking verbs:
* be (am, is, are, was, were, been, being)
* become
* seem
* appear
* feel
* look
* sound
* taste
* smell
"A" is an indefinite article, used before singular countable nouns. It doesn't have the grammatical function of a linking verb.
For example:
* "He is a doctor." ("is" is the linking verb, connecting "He" to "a doctor")
* "She is a good student." ("is" is the linking verb, connecting "She" to "a good student")
"A" is simply part of the subject complement in these sentences.