What is a complete subject in the sentence- It fun to go swimming on hot day?

There is no complete subject in the given sentence. The subject should be a noun or pronoun that performs the action of the verb. In this case, the verb is "fun," but there is no noun or pronoun that is performing the action. Therefore, the sentence is grammatically incorrect and incomplete.

To correct it, we can add a subject like "Swimming" to make it a complete sentence: "Swimming is fun on a hot day."