You're asking about essential nouns, pronouns, or groups of words that act as nouns (called noun phrases) that are necessary for a sentence to make sense.
These are often called the subject of the sentence.
Here's why they are essential:
* Subject-Verb Agreement: The subject tells us who or what is performing the action in the sentence. The verb must agree with the subject in number (singular or plural).
* Meaning: Without a subject, we don't know *who* or *what* is doing the action, making the sentence incomplete and nonsensical.
Examples:
* The dog barked loudly. (The subject is "the dog")
* She walked to the park. (The subject is "she")
* The big, red ball bounced down the stairs. (The noun phrase "the big, red ball" is the subject)
Let's test it:
* Barked loudly. (This sentence doesn't make sense without a subject.)
* Walked to the park. (This sentence doesn't make sense without a subject.)
* Bounced down the stairs. (This sentence doesn't make sense without a subject.)
Therefore, the essential noun, pronoun, or noun phrase acting as the subject of a sentence cannot be left out.