未來式存在句

照理說應該是這樣沒錯
There +will+be+名詞+ 地點 哪裡會有~
如：There will be many students in the park tomorrow 明天公園裡會有很多學生

這句我想應該是因為a lot of fun的關係
因為通常會說 it will be a lot of fun 一定會很好玩
所以句子就變成 The birthday party will be a lot of fun

如果你要說there will be a lot of fun in the party應該也沒錯，但似乎有點不自然

翻成中文的話
第一句會是：明天的生日趴一定很好玩
第二句；明天的生日趴一定有很多樂趣

或許兩句都沒有錯，只是表達的方式不同，我個人是覺得第一句比較自然。

Do you also feel weird of the two following sentences?
1. It could be fun, but I don't feel like going out this late.
2. Come on! It will be a lot of fun.

2014-01-12 16:47:16 補充：
fun 除了可以當成名詞：『樂趣』之外， 還可以當成形容詞：『有樂趣的』。
It will be a lot of fun.
文法上來說，a lot of 是形容詞，所以這裡的fun應該是名詞，那這一整句就會有點奇怪，好像在說，it = fun。所以會覺得應該要用 There will be a lot of fun. 才合理。

2014-01-12 17:00:15 補充：
It can be fun. 這裡的 fun是形容詞，所以整句讀起來就沒有問題。
但是如果我們把『It will be a lot of fun』當成關鍵字搜尋，又會發現有非常多人是這樣使用。怎麼解釋呢？ I don't know. 也許文法專家有好的見解，或者可能又會說這是母語人士錯誤的口語用法，或是這是個例外。Anyway, as long as it is widely used and it doesn't cause any confusion, it will be ok to me.

2014-01-12 17:36:12 補充：
Maybe in this sentence, "It will be a lot of fun.", it means "Participating in the birthday party" will be a lot of fun.

2014-01-13 01:22:34 補充：
沒聽過什麼是存在句. 英語就英語, 哪來那麼多專有名詞....Orz..
不過回到你的問題: ....為什麼不是.....?
一句話本來就不是只有一種講法.
你是要問這個嗎?

Using "be" in "The birthday party tomorrow will be a lot of fun" is like "I am fun", kind of weird. In this sentence "The birthday party tomorrow will be a lot of fun" might not be a problem. However, I would use "have" to replace "be".

2014-01-12 11:11:36 補充：
Bath! Like I said, it MIGHT not be a problem! However, I understand your point.

There +will +be 其中 There 是 主詞
The birthday party 也是主詞 所以 也沒錯呀！！ ZZZZZZ