grammar - Is よう an adjective or a noun?

I have learnt the pattern Noun + のような. My question is よう is a Noun or an Adjective. If it is an Adjective, then why it is preceded by a の. If it is a Noun then why it is followed by a な. What is behind this pattern? Thanks for your help.

Answer

Is よう an adjective or a noun?

Yes. :)

More detailed examination...

よう, or in kanji 様, is a bit of an odd duck. It functions in some ways as a 名詞【めいし】 or a noun on its own, but when used in certain constructions, it takes on qualities of a 形容【けいよう】動詞【どうし】 or -na adjective.

Japanese grammars often classify this term as a 助動詞【じょどうし】 or auxiliary, which is basically a grab-bag of very-useful functional words that don't fit cleanly into the other grammatical categories. If you can read Japanese, there's a decent article on Wikipedia. Even if you can't read Japanese that well, the big table there should give you some idea of the wide variety of things that fall into this Big Bucket o' Vague Categorization.

For most Japanese learners, it might help to think of よう as a noun that has developed some -na adjective uses. Very loosely, it's a bit like the English nouns "kind" or "sort", which have developed extended uses in phrasing like "kind of / kinda", "kinda like", "sort of / sorta", "sorta like" -- these function like almost-adjectives deriving from nouns, and are used to convey the general sense or "that-ness" of a thing or situation.

Your additional questions

If it is an Adjective, then why it is preceded by a の?

Again, this word started out as a noun, much like English "sort" or "kind". To modify any noun in Japanese, you need to either use a verb or adjective -- like 来【き】たよう, or 暖【あたた】かいよう -- or if the preceding word is a noun, you need to use that linking の -- like 犬【いぬ】のよう, or 駐【ちゅう】車【しゃ】場【じょう】のよう.

If it is a Noun then why it is followed by a な?

If that よう is used to modify something else, it needs a linking particle. Nouns usually require the particle の to link to (i.e. modify) another noun. That said, historically, certain nouns and noun-like words that were used as descriptors for other nouns developed a different particle pattern, which evolved into the modern な that's used with -na adjectives. よう is one of these words.

Note that よう doesn't require the な all the time -- that's only needed when the よう modifies another noun. If it's used as the main point of the sentence, then it doesn't need the な.

• 田中【たなか】さんが到着【とうちゃく】した[よ]{●}[う]{●}です。
  Tanaka-san [SUBJ] arrival did appearance is
  It seems that Tanaka has arrived.

But if the よう is used to modify another noun, then we need the な.

• この森【もり】は公園【こうえん】の[よ]{●}[う]{●}[な]{●}雰囲気【ふんいき】があります。
  This forest [TOPIC] park [POSSESSIVE] appearance [MODIFIER] atmosphere [SUBJ] is/has
  This forest has an atmosphere kinda like a park.

This is a very useful word in Japanese that doesn't have any one-to-one match in English. As you can see above, there are various ways of translating the meaning of よう.

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Please comment if the above does not fully answer your questions, and I can update as appropriate.