Here's why:
* Subject: The subject is the noun or pronoun that performs the action of the verb. It's who or what the sentence is about.
* Complete Subject: The complete subject includes the subject and any words that modify or describe it.
Without a subject, you wouldn't know who or what is doing something. The sentence wouldn't be complete and would be grammatically incorrect.
Examples:
* Incorrect: Ran quickly. (Who ran?)
* Correct: The dog ran quickly. (The dog is the complete subject)
Exceptions:
There are a few rare cases where a sentence might *seem* to lack a subject, but it actually has an implied subject.
* Imperative sentences: These sentences give commands and usually assume the subject is "you." For example, "Go to the store." (You go to the store)
* Sentences with "there" as a placeholder: "There are many birds." (Many birds are there)
However, even in these cases, the implied subject is still present.