How to tell when a na adjective is being used as a noun (as opposed to an adjective)

Question

It is common knowledge that na-adjectives are in truth nouns that can be used as adjectives (edit: I now see this was a false generalization I was led into believing by some grammar sources); and the commonest formula where it's obvious the na-adjective is used as an adjective is "na-adjective+な+noun". But when they are used as the predicate in noun sentences (i.e. "na-adjective+だ/です") I can never be sure if what is meant is a noun, or an adjective.

For example, below are examples where a na-adjective is used as a predicate:

これは幸せです。-> Does this mean "This is happiness" OR "This is happy (this is a happy occassion)

空はきれいです。-> Does this mean "The sky is beautiful" OR "The sky is beauty (the sky is the definition of beauty)"?

夜は静かです。-> Does this mean "The night is silent" OR "The night is silence (in a poetic sense)"

While in the above examples, the na-adjectives are grammatically nouns; most of the time in the English translation, they are adjectives. I'm assuming the first translations I've given above would be the commonest choice of meaning. However, in the case of 幸せ, what is meant is a noun; in the case of きれい and 静か, what is meant is an adjective. How does one tell if a na-adjective used as a predicate is meant to be an adjective and not a noun, or vice versa?

Is it simply a matter of how often they are used as nouns when used without な? (i.e. do we translate 幸せです as "this is happinnes" as opposed to "this is happy", simply because 幸せ is very often used as a noun?)

Answer 1

Some -na adjectives can also be used validly and grammatically as nouns. Your example sentences happen to include -na adjectives that cannot be used as nouns.

• 綺麗【きれい】 is only ever an adjective. The noun form would be 綺麗さ.
  • Nihon Kokugo Dai Jiten (NKDJ) entry at Kotobank (in Japanese). Note that the part-of-speech marker at the top only has 『形動』 (short for 形容動詞【けいようどうし】 or "-na adjective").
  • Digital Daijisen (DDJS) entry at Kotobank (in Japanese). Note the same part-of-speech marker. Note too the 『[派生]きれいさ［名］』 line towards the bottom, showing that the noun derivative is formed by adding ～さ on the end.
• 静【しず】か, outside of extremely rare exceptions such as 「静かの海」 as noted by @broccoli facemask in the comments, is only ever an adjective.
  • NKDJ entry at Kotobank -- this one includes the noun derivative towards the bottom of the page.
  • DDJS entry at Kotobank

In these cases, even when used predicatively, we know that these words can't be nouns unless they include the nominalizing suffix ～さ, so constructions like 「あの子は綺麗です」 or 「この森は静かです」 can only be understood as [NOUN = ADJ] sentences.

In those cases where a -na adjective can also be used as a noun, such as 幸【しあわ】せ, context can help you figure out the intended sense.

• 幸【しあわ】せ
  • NKDJ entry at Kotobank, showing a part of speech of 〘名〙, but then also including examples with clear -na adjective constructions.
  • DDJS entry at Kotobank, showing a part of speech of ［名・形動］.

Your sample sentence:

• これは幸せです。
  • We don't have a lot of context here. That said, it's not that common that people talk about something being "happiness itself", so this is most likely saying that これ is 幸【しあわ】せ as an adjective: "happy, fortunate, lucky".

⇒ Beware of the pitfalls of translation, however! While it may be valid to parse this sample sentence as [THIS = NOUN], we have to think about how Japanese fits together conceptually. Considering the senses of 幸【しあわ】せ, in terms of "things coming together in a favorable way", even were we to use the noun interpretation, it's probably a better translation to think of this as "an instance of things coming together in a favorable way", rather than any kind of over-arching meaning of "the essence of favorable coincidence in any and all circumstances".

Looping back to your core question:

How does one tell if a na-adjective used as a predicate is meant to be an adjective and not a noun, or vice versa?

1. Check the dictionary for the part of speech for the word.
   • Not all -na adjectives can be used as nouns.
2. Check the dictionary for details on the meaning.
   • If the word can be used as a noun, does the noun make sense in your context?
3. Check usage patterns.
   • Does the meaning you're thinking of still work when the term is used in other sentences? Google, Linguee, and other search engines can be your friend in this regard.