Here's why:
* Linking verbs connect the subject of a sentence to a noun or adjective that renames or describes it. Examples include: *be, seem, appear, become, feel, look, sound, taste, smell*.
* Modal auxiliary verbs like *must* express a possibility, necessity, or permission. They always accompany another verb.
* Helping verbs assist the main verb in expressing a specific tense, aspect, or voice.
In the phrase "must have," *must* is the modal auxiliary and *have* is the helping verb. Together, they help form the verb phrase.
For example, in the sentence: "You must have missed the bus," the verb phrase "must have missed" expresses a past possibility or necessity.
Let me know if you'd like more examples!