What is the simple subject of sentence a large part park underwater?

The simple subject of the sentence "A large part of the park is underwater" is part.

Here's why:

* Subject: The subject of a sentence is the person, place, thing, or idea that the sentence is about.

* Simple Subject: The simple subject is the main noun or pronoun that performs the action of the verb.

In this case, the sentence is about a "part" of the park. While "large" and "park" are modifiers, they don't perform the action of the verb "is."