Catching fish is one of the oldest pastimes.

what part of speech is fish in the sentence.

i think it's noun.

Yes, it's a noun. It's serving as the direct object of the gerund "catching."

Large fish swim swiftly in the sea.


Large is an adjective in this sentence right?

Yes.

Right.

Catching fish is one of the oldest pastimes.

( is )in the sentence is a preposition
correct

Nope.

Is is always a verb.

You are correct! In the sentence "Catching fish is one of the oldest pastimes," the word "fish" is indeed a noun. It refers to a type of aquatic animal and functions as the object of the verb "catching." In order to determine the part of speech of a word in a sentence, you can identify its role and function within its specific context.