Learn Swedish: Bollen är stor och spelet är roligt. - The ball is big and the game is fun.

This is because adjectives must match the gender of the noun they describe. Boll (ball) is an en-word, so stor stays in its base form. Spel (game) is an ett-word, so we must add a -t to the adjective, making it roligt.

How can I tell from this sentence that boll is an en-word and spel is an ett-word?

You can look at the endings of the nouns! "The ball" is bollen, which ends in -en, telling us it is an en-word. "The game" is spelet, which ends in -et, showing that it is an ett-word.

Does roligt mean "fun" or "funny"?

It can actually mean both! Depending on the context, rolig (or roligt) translates to either "fun" or "funny". Here, "the game is fun" makes the most sense.

Could I say Det är roligt to just mean "It is fun" in general?

Yes! When you talk about a general situation or an activity rather than a specific noun, Swedish uses the impersonal det (it). Because det acts like an ett-word, the adjective always takes the -t ending in these cases, just like it does for spelet.