Here's why:
* Linking verbs connect a subject to a noun, pronoun, or adjective that renames or describes the subject. Examples include: *is, am, are, was, were, be, being, been, seem, appear, become, feel, look, smell, taste, sound*.
* "Must" is a modal auxiliary verb. Modal verbs help express possibility, necessity, obligation, and permission. They always appear before the main verb, adding a specific nuance to the sentence's meaning.
Example:
* Incorrect: "The cake must delicious." ("Must" is being used as a linking verb, but it doesn't connect the subject to a word that renames or describes it.)
* Correct: "The cake must be delicious." ("Must" is used as a modal auxiliary verb, indicating the speaker's certainty about the cake's taste.)
Let me know if you have any other questions about linking verbs or modal auxiliaries!