Here's why:
* Helping verbs assist the main verb: They help to create different tenses, moods, or voices. For example, "is" in "He is running" helps to form the present continuous tense.
* Subjects are the actors of the sentence: They are the nouns or pronouns that perform the action of the verb.
* Helping verbs are part of the verb phrase: They work alongside the main verb to convey meaning.
Example:
* Sentence: "The dog has been sleeping all day."
* Subject: "The dog"
* Verb Phrase: "has been sleeping"
* Helping Verb: "has"
* Main Verb: "sleeping"
Therefore, helping verbs are always part of the verb phrase, not the subject.