Why would you use な for そう forms of い adjectives?

I am just now really diving into adjectives and how they work in Japanese. It was to my surprise that I heard 『楽{たの}しいそうなお祭{まつ}りね』 being used. Does the そう usage require な even for い adjectives?

Answer

There are two things I would like to do as part of the answer to your question.

First, you may not have been aware that there was a mistake in your sentence. I will address that first.

You wrote:

楽{たの}しいそう

It should be:

楽{たの}しそう

For the formation of the そうだ grammar is as follows:

• Vmasu + そうだ


• adj(i/na) stem + そうだ

Examples:

話しそう (verb)
楽しそう (i adjective)
静かそう (na adjective)

_{Source: A Dictionary of Basic Japanese Grammar (See below)}

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Now for the question you acked. According to A Dictionary of Basic Japanese Grammar (Seiichi Makino and Michio Tsutsui):

そうだ is a な-type adjective; The prenominal form is そうな. Examples:

高そうな車
taka souna kuruma
(A car which looks espensive (= an expensive looking car))

雨が降りそうな空
ame ga furi souna sora
(lit. the sky which looks like it will bring rain)

So to directly answer your question:

Does the そう usage require な even for い adjectives?

The answer is yes. Note that most of the time, you will find this formation at the end of a sentence requiring it to end with だ/です. However, if you plan on using this grammar as an adjective (i.e. you mean to use it to modify a noun), you should treat is as a na-type adjective.

_{Note: I included a link to the amazon listing of the book I used for my source. If you plan to purchase it, look around. I have definitely seen it sell for less.}