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 ' EQUALS '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 )

In the above examples, teachers and unwell are subject complements.