Learn Hardcore Japanese: yama de yozora no hosi wo miruno ha tanosii desu. - It is fun to look at the stars in the night sky in the mountains.

山で = in/at the mountains (place where you do something)
山に is more like to the mountain(s) (destination) or at the mountain(s) in the sense of “existing there,” not “doing something there.”

Because the sentence is about doing an activity (見る – to look) in the mountains, で is the natural choice:

山で星を見る = (I) look at stars in the mountains.

What does 夜空の星 literally mean, and why is の used between 夜空 and 星?

Literally, 夜空の星 is:

夜空 – night sky
の – “of” / belonging to / modifying
星 – stars

So it’s “stars of the night sky” or more naturally “the stars in the night sky.”

In Japanese, の between two nouns often works like English “of” or like a noun-as-adjective:

夜空の星 = the night sky’s stars / night-sky stars

You could also say 夜の空の星 (“stars of the night sky”), but 夜空 is a fixed, single word, so 夜空の星 is shorter and more natural.

What is を doing in 夜空の星を見る?

を marks the direct object of a verb — the thing the action is done to.

夜空の星を – the stars of the night sky (as the object)
見る – to look at / watch

So 夜空の星を見る = “(to) look at the stars in the night sky.”

Even when the whole phrase becomes a noun with の (見るの), the internal structure is still:

[夜空の星を] (object) + [見る] (verb)

What is the の doing in 見るの?

Here の is a nominalizer: it turns a verb phrase into a noun-like phrase, like English “watching / to watch / the act of watching.”

見る – to see / to look
見るの – seeing / looking (as a thing, an activity)

So:

夜空の星を見るの ≈ watching the stars in the night sky (as an activity)

This entire chunk 夜空の星を見るの can now function grammatically like a noun, so it can be followed by は, が, etc.

Why is it 見るのは and not 見るのが? What’s the difference between V-のは and V-のが?

Both patterns are possible in many sentences:

V-のは
- adjective
V-のが
- adjective

Basic nuance:

のは tends to mark a topic (“as for …”) or contrast.
のが tends to mark what specifically is something (more neutral focus).

In this sentence:

山で夜空の星を見るのは楽しいです。

This feels like a general statement:

As for watching the stars in the night sky in the mountains, (it) is fun.

If you said:

山で夜空の星を見るのが楽しいです。

that’s more like:

The thing that is fun is watching the stars in the night sky in the mountains.

Both are natural here; は just sounds a bit more like a general observation.

Could we say 見ることは instead of 見るのは? What’s the difference between の and こと here?

Yes, you could say:

山で夜空の星を見ることは楽しいです。

It’s grammatically fine and means essentially the same thing.

Nuance:

V-の: often feels a bit more casual, concrete, or personal.
V-こと: often feels slightly more formal, abstract, or written.

In practice, both are used in neutral sentences like this. In everyday speech, V-のは楽しい may feel a touch more natural, but V-ことは楽しい is not wrong.

What does 楽しいです express, and how is it different from 楽しみです?

楽しい is an adjective meaning fun / enjoyable.
- 楽しいです = “(it) is fun.”
楽しみ is a noun meaning pleasure / something to look forward to / anticipation.
- 楽しみです ≈ “I’m looking forward to it / It’s something I’m excited about.”

So:

山で夜空の星を見るのは楽しいです。
= The activity itself is fun.

If you said something like:

山で夜空の星を見るのが楽しみです。
= I look forward to watching the stars in the mountains.

Different nuance: current fun vs looking forward to it.

Why isn’t there an explicit subject like “I” or “we” in the sentence?

Japanese often omits the subject when it’s clear from context.

In English we must say “I think it’s fun”, “We think it’s fun”, or “It is fun”.
In Japanese, if it’s obvious who feels it’s fun, you just leave it out.

If you really want to specify:

私は山で夜空の星を見るのは楽しいです。
“As for me, watching the stars in the night sky in the mountains is fun.”

But in natural Japanese, 私 is often unnecessary unless there’s ambiguity or contrast.

Is the word order fixed? Can I move 山で or 夜空の星 to other positions?

Japanese word order is relatively flexible as long as the particles are correct. For example:

山で夜空の星を見るのは楽しいです。 (original)
夜空の星を山で見るのは楽しいです。

Both are grammatical. The basic meaning is the same, but the default and most natural order is location (山で) before the object (夜空の星を).

You generally keep:

modifiers before the words they modify
the main verb or predicate at the end

but within that, elements marked by particles can be reordered.

Could we say 山の夜空の星 instead of 山で夜空の星? What would change?

山で夜空の星を見る:
Focuses on the place where you watch them — you are in the mountains doing the watching.
山の夜空の星を見る:
Now 山の modifies 夜空 – “the mountain’s night sky,” so literally “the stars of the mountain night sky.”
It emphasizes the night sky that belongs to / is over the mountains, rather than the location of you.

Both are possible, but:

山で夜空の星を見る is more straightforward and natural when you just mean “looking at the night sky’s stars while in the mountains.”
山の夜空の星 sounds a bit more literary or descriptive of the sky itself.

How does です change the sentence? What’s the difference between 楽しい and 楽しいです?

楽しい – plain form; informal, used with friends, in casual writing, or in neutral dictionary examples.
楽しいです – polite form; adds politeness, appropriate for talking to people you’re not close to, in class, etc.

So:

山で夜空の星を見るのは楽しい。 – casual / plain
山で夜空の星を見るのは楽しいです。 – polite

The meaning (“is fun”) is the same; it’s just a matter of politeness level.

How would I say this in the negative or past tense while keeping the same structure?

You keep the same pattern V-のは + adjective + です and change the adjective:

Negative (not fun):
- 山で夜空の星を見るのは楽しくないです。
  = Watching the stars in the night sky in the mountains is not fun.
Past (was fun):
- 山で夜空の星を見るのは楽しかったです。
  = Watching the stars in the night sky in the mountains was fun.
Past negative (was not fun):
- 山で夜空の星を見るのは楽しくなかったです。

Is this pattern “V-のは + adjective + です” a general way to say “Doing X is ADJECTIVE” in Japanese?

Yes. This is a very common and useful pattern:

[Verb in dictionary form] + のは + [adjective] + です

Examples:

日本語を勉強するのは難しいです。
= Studying Japanese is difficult.
音楽を聞くのは楽しいです。
= Listening to music is fun.
朝早く起きるのはつらいです。
= Getting up early in the morning is tough.

Your sentence fits exactly into this pattern:

山で夜空の星を見るのは楽しいです。
= Watching the stars in the night sky in the mountains is fun.