Noun+Adjective - What are the rules/usages?

Question

I usually self-study, however I see a tutor once a week who is a native from Tokyo. In my previous lesson with her, she used the following:

もう夜遅いから家に帰ったほうがいいでしょう。

Where 夜遅い translates to "late at night", and the overall translation is something like "Since it's already late, we should head back home."

When I asked her if there are rules governing this construct, she was not able to give me a definitive answer, but instead was only able to give me one further example:

朝早い = Early in the morning

I am wondering, are these two examples just set expressions, or are there rules that govern what noun+adj combinations are possible? Is it strictly used with [unit of time (Noun)]+[qualitative (Adj)], or is there a wider range of usage?

Answer 1

There actually is a rule governing the "Noun + Adjective" structures.

The noun must signify either a spatial or temporal unit such as [朝]{あさ}、[夜]{よる}、[空]{そら}、[天]{てん}、[奥]{おく}、 [時]{とき}、 [数]{かず}, etc.

The adjective must signify a degree or quantity regarding the preceding noun. These include [早]{はや}い、[遅]{おそ}い、[高]{たか}い、[低]{ひく}い、[多]{おお}い、[少]{すく}ない、[深]{ふか}い, etc.

Thus, it is completely natural and correct to combine these nouns and adjectives in the "noun + adjective" order without using particles.

Lastly, since adjectives do conjugate in Japanese, these expressions are also often used as adverbial expressions modifying verb phrases.

Examples of adjectival usages: 夜遅い、朝早い、空高し、数多い、時遅し, etc.

Examples of adverbial usages: 夜遅く、奥深く、数少なく、天高く、朝早く, etc.

Answer 2

I believe this is just an example of particle omission. Particles are often omitted if it's clear from context. In this case, the particle omitted is が. That is to say, the fully expression would be:

夜(が)遅い

朝(が)早い

There's no hard fast rule specific to time of day and temporal adjective (there are guidelines). Particle omission is something that you have to learn when and how to do it by experience. If you're not fully comfortable with it or you have any doubts on whether or not it's okay to omit, you should keep the particle to be on the safe side.