Learn Hardcore German: Der Sternenhimmel über dem See ist wunderschön. - The starry sky over the lake is wonderfully beautiful.

Sternenhimmel is masculine: der Sternenhimmel.
In this sentence, Der Sternenhimmel is the subject (the thing that is beautiful).
Subjects in German normally take the nominative case, so you use der, not den (accusative) or dem (dative).

So: Der Sternenhimmel = The starry sky (subject).

What is Sternenhimmel exactly? Is it a compound noun?

Yes, Sternenhimmel is a compound noun:

der Stern = star
der Himmel = sky / heaven

Put together: der Sternenhimmel = literally star-sky, usually translated as starry sky or starlit sky.

Because it’s a noun, it is capitalized in German.
Its plural is rare, but would be die Sternenhimmel.

Why is it über dem See and not über den See?

The preposition über can take either dative or accusative, depending on the meaning:

Dative (location, where? – no movement):
- über dem See = above/over the lake (static position)
Accusative (direction, where to? – with movement):
- über den See = over/across the lake (movement from one side to the other)

In your sentence, the starry sky is simply located above the lake; it’s not moving across it.
So dative is used: über dem See.

Why is it dem See and not der See after über?

Der See (the lake) is masculine:

Nominative: der See
Accusative: den See
Dative: dem See

After über with a static location, we use the dative case.
So: über dem See, not über der See or über den See.

Note: There is also die See (feminine), which usually means the sea/ocean, but here we are clearly talking about der See = the lake.

Could the word order be Über dem See ist der Sternenhimmel wunderschön?

Yes, that word order is grammatically correct:

Der Sternenhimmel über dem See ist wunderschön. (neutral, subject-first)
Über dem See ist der Sternenhimmel wunderschön. (emphasis on the location)

By moving Über dem See to the front, you emphasize the place more strongly: Over the lake, the starry sky is beautiful (there, specifically).

German allows fairly flexible word order as long as the finite verb (ist) stays in second position.

Why doesn’t wunderschön have an ending like wunderschöne?

German adjectives behave differently depending on where they stand:

Attributive (directly before a noun): they take endings
- ein wunderschöner Sternenhimmel (a beautiful starry sky)
- der wunderschöne Sternenhimmel
Predicative (after sein, werden, bleiben, etc.): no ending
- Der Sternenhimmel … ist wunderschön.
  (The starry sky is beautiful.)

In your sentence, wunderschön is predicative (linked to the subject by ist), so it stays in its basic form, without an ending.

What is the nuance of wunderschön compared to schön or sehr schön?

schön = beautiful, nice, pretty
sehr schön = very beautiful
wunderschön = wonderfully beautiful, gorgeous, stunning

wunderschön is stronger and more emotional than just schön; it’s close in feeling to very beautiful, absolutely beautiful, or gorgeous in English.

Could I say Der Sternenhimmel am See ist wunderschön instead of über dem See?

You could, but the meaning changes slightly:

über dem See = above the lake
- Focus: the sky over that body of water.
am See (an + dem) = at/by the lake
- Focus: the location of the observer or general area; sounds more like
  The starry sky at the lake is beautiful (when you’re at the lake looking up).

Both are possible, but über dem See paints the picture of the sky hanging above the lake more clearly.

Can I drop the article and say Sternenhimmel über dem See ist wunderschön?

No, that sounds unnatural in standard German.

In German, singular countable nouns almost always need an article (definite or indefinite) unless used in certain fixed expressions or headlines.

So you normally say:

Der Sternenhimmel über dem See ist wunderschön.

Without der, it sounds like a note or label, not a normal sentence.

How do I pronounce Sternenhimmel and über?

Approximate pronunciation (IPA and rough English hints):

Sternenhimmel: /ˈʃtɛʁnənˌhɪməl/
- St at the beginning: like sht in *sht*orm
- e in Stern: like e in bed
- himmel: roughly like HIM-mel, both m and el clearly pronounced
über: /ˈyːbɐ/
- ü: a rounded sound between ee in see and u in dude; you round your lips while saying something like ee
- Final -er in standard German: /ɐ/, a kind of weak “uh” sound

So the full sentence roughly:

Der Sternenhimmel über dem See ist wunderschön.
/deːɐ̯ ˈʃtɛʁnənˌhɪməl ˈyːbɐ deːm zeː ɪst ˈvʊndɐˌʃøːn/

Why is the verb ist and not something else like gibt or hat?

Here, you are describing a state, not stating existence or possession:

ist (from sein) = is → used to link the subject to a description
- Der Sternenhimmel … ist wunderschön.
  The starry sky … is beautiful.

Compare:

Es gibt einen Sternenhimmel über dem See.
There is a starry sky above the lake. (existence)
Der See hat einen wunderschönen Sternenhimmel.
The lake has a wonderful starry sky. (possession-like)

Your sentence is a description, so sein (ist) is the natural choice.