Is it grammatically correct to say was so fun?

"Was so fun" is not grammatically correct on its own. "Fun" is an adjective, and it needs a noun to describe.

Here are some correct ways to use "fun":

* "That was so much fun!" (This is the most common way to express that something was enjoyable.)

* "The party was so fun." (Here, "fun" describes the noun "party".)

* "We had so much fun." (Here, "fun" describes the noun phrase "so much fun".)

Let me know if you'd like to see more examples!