Here's why:
* Linking verbs connect the subject to a noun or adjective. They don't express an action. For example, "The cat *is* fluffy." "Is" links the subject "cat" to the adjective "fluffy".
* Sentences need a predicate. The predicate is the part of the sentence that tells us what the subject does or is. A linking verb alone doesn't provide this information.
To have a complete sentence with a linking verb, you need:
1. Subject: The person, place, or thing the sentence is about.
2. Linking Verb: Connects the subject to the predicate.
3. Predicate: This is the part that describes the subject. It can be a noun, adjective, or pronoun.
Example:
* Subject: The sky
* Linking Verb: is
* Predicate: blue
Complete Sentence: The sky is blue.
In short, linking verbs need a complement (noun, adjective, pronoun) to complete the sentence.