Here's why:
* Linking verbs connect the subject to a word or phrase that renames or describes the subject. This word or phrase is the subject complement.
* Common linking verbs: *be* (am, is, are, was, were, been, being), *become*, *seem*, *appear*, *feel*, *look*, *sound*, *smell*, *taste*, *remain*, *stay*
Examples:
* The sky is blue. (The subject complement "blue" describes the subject "sky")
* She became a doctor. (The subject complement "a doctor" renames the subject "she")
* The music sounds beautiful. (The subject complement "beautiful" describes the subject "music")
Exceptions:
* Sentences with an intransitive linking verb: Some linking verbs (especially *be*) can function without a subject complement.
* He is here. (No subject complement)
* They are gone. (No subject complement)
Important Note: While linking verbs are almost always accompanied by a subject complement, there are rare exceptions where a sentence might contain a linking verb but not a subject complement.