Learn Hardcore Japanese: asita ha yakyuutaikai no kessyou ga aru. - Tomorrow there is the final game of the baseball tournament.

は is the topic marker and signals “as for tomorrow…,” setting the context. が would instead mark the grammatical subject. By choosing は, the speaker emphasizes “tomorrow” as the topic and then comments on it (i.e. “there is a baseball tournament final”).

What does the particle の do between 野球大会 and 決勝?

The particle の links two nouns in a genitive or descriptive relationship. Here it turns 野球大会 (“baseball tournament”) into a modifier of 決勝 (“final”), producing “the final of the baseball tournament.”

Why is が used before ある in 決勝がある?

In sentences with the existential verb ある (“there is” for inanimate things), が marks the thing that exists. So 決勝がある means “the final exists” or “there is a final.”

What nuance does ある carry here? Is it different from “to have”?

ある expresses existence or occurrence of an inanimate event or object—“there is/are.” It’s not a possession verb. Thus 決勝がある means “the final will take place.”

Why is the word order 明日は野球大会の決勝がある instead of the English “Tomorrow the baseball tournament’s final there is”?

Japanese typically follows Topic–Object–Verb (SOV) order. You state the topic (明日は), then the modified noun phrase (野球大会の決勝), mark the subject with が, and end with the verb (ある).

Could this be made more polite?

Yes. You could use the polite form あります:
明日は野球大会の決勝 があります。
That raises the politeness level to standard polite speech.

What’s the pronunciation and meaning of 決勝?

決勝 is pronounced けっしょう (kesshō). It means “decisive match” or “final.”

Why not say 野球大会の決勝戦 instead of 決勝?

You can. 決勝戦 (けっしょうせん) literally adds 戦 (“match/fight”) to emphasize it’s a game. Both are common, though 決勝 alone is sufficient to mean “the final.”

Is it possible to drop は after 明日 here?

In very casual speech you might omit 明日’s particle:
明日, 野球大会の決勝がある。
But in writing or careful speech you’d keep は to clearly mark the topic.