(something) is a lot of fun

When you're talking about "fun", don't use "very" and "not very". Instead, think of "fun" as having an amount, like water. Use "a lot of fun", "not much fun", "so much fun", "not any fun", etc.:

I had so much fun last night.

It's a lot of fun studying learning to speak to people in a foreign language.

I remember Disney World not being very much fun because we had to stand in such long lines for everything.

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