Linking verbs have NO object. Linking verbs link two parts of a sentence. They link the subject to a noun or adjective. In this sense, linking verbs are like a mathematical equals sign .
Example : Mary is a nurse. ( Mary EQUAL a nurse )

Linking verbs do not make sense if used alone: they need a "subject complement" to complete their meaning.
Example
1. They are - They are teachers
2. I feel - I feel unwell