Learn Hardcore German: Der Sternenhimmel sieht heute so lebendig aus, dass wir lange draußen bleiben wollen. - The starry sky looks so lively today that we want to stay outside for a long time.

The infinitive is aussehen (to look / to appear).
In a main clause in the present tense, separable verbs split:
- The finite verb (conjugated part) goes in position 2.
- The separable prefix goes to the end of the clause.

So:

Der Sternenhimmel sieht heute so lebendig aus.
- sieht = 3rd person singular of sehen
- aus = separable prefix

If you move to a context where the verb is not in second position (infinitive, subordinate clause, perfect tense), the verb is written together again:

Er möchte den Sternenhimmel sehen. (infinitive, not separable here)
…, weil der Sternenhimmel heute so lebendig aussieht. (subordinate clause)
Der Sternenhimmel hat heute so lebendig ausgesehen. (perfect tense)

Could I also say Der Sternenhimmel ist heute so lebendig instead of sieht … so lebendig aus?

You can, but the nuance is slightly different.

Der Sternenhimmel ist heute so lebendig.
- Uses sein (to be).
- Sounds like an inherent state or quality.
Der Sternenhimmel sieht heute so lebendig aus.
- Uses aussehen (to look, to appear).
- Emphasizes how it appears to you, what it looks like.

In everyday speech, both are fine, but sieht … aus focuses more clearly on the visual impression (what you see in the sky).

What is the structure so … dass doing in this sentence?

So … dass expresses a degree that leads to a consequence, similar to English:

so … that …

In the sentence:

so lebendig, dass wir lange draußen bleiben wollen
→ so lively that we want to stay outside for a long time

Grammar points:

so + adjective/adverb: so lebendig (so lively)
Comma
dass introduces a consequence clause (subordinate clause)
The verb in the dass-clause goes to the end:
- … dass wir lange draußen bleiben wollen.

You can use this pattern with many adjectives:

Es ist so kalt, dass ich zittere. (so cold that I’m shivering)
Er spricht so schnell, dass ich nichts verstehe. (so fast that I don’t understand anything)

Why do we use dass here and not weil?

dass = that, introducing a result/consequence clause.
weil = because, introducing a reason clause.

Here, the liveliness of the starry sky causes a reaction, a consequence:

The sky is so lively → as a result, we want to stay outside.

So we use so … dass (so … that …), not weil (because).

If you used weil, you’d get a different structure:

Wir wollen lange draußen bleiben, weil der Sternenhimmel heute so lebendig aussieht.
→ We want to stay outside for a long time because the starry sky looks so lively today.

This is logically similar, but grammatically the relationship is reversed:
Main clause = wanting to stay outside; weil-clause = reason.

Why is there a comma before dass?

In German, every subordinate clause is separated from the main clause by a comma.

Der Sternenhimmel sieht heute so lebendig aus,
→ main clause
dass wir lange draußen bleiben wollen.
→ subordinate clause, introduced by dass

So the comma is obligatory.

Same rule with other conjunctions that form subordinate clauses:

…, weil …
…, obwohl …
…, wenn …
…, als …, etc.

What’s going on with the word order wir lange draußen bleiben wollen in the dass-clause?

This is typical subordinate-clause and modal-verb word order.

Rules:

In a subordinate clause (introduced by dass), the finite verb goes to the end.
With a modal verb (here wollen) + another verb (bleiben), the full verb (bleiben) goes before the modal at the very end of the clause.

So we get:

dass = subordinating conjunction
wir = subject
lange = adverb of time
draußen = adverb of place
bleiben = full verb (infinitive)
wollen = modal verb (finite, so it must be last in a subordinate clause)

Structure:
dass + Subject + (other elements) + full verb + modal verb

Other examples:

…, dass wir früh nach Hause gehen müssen.
…, dass sie morgen länger bleiben will.

Why is it wollen and not möchten here?

Both can translate as want (to) in English, but:

wollen = to want (strongly/intentionally), to plan/aim to do something.
möchten = more polite/soft, often like would like to.

In this sentence:

… dass wir lange draußen bleiben wollen.
→ We (firmly) want to stay outside for a long time.

If you said:

…, dass wir lange draußen bleiben möchten.
→ We would like to stay outside for a long time.

Both are grammatically fine; wollen sounds a bit more determined or simply neutral depending on context. Möchten is more polite or cautious.

Why is it wir and not uns in the second clause?

Because wir is the subject of the subordinate clause and must be in the nominative case.

wir = nominative (subject form)
uns = accusative/dative (object forms)

In dass wir lange draußen bleiben wollen:

wir = the people who want and who stay → subject
There is no object that would require uns.

So wir is the only correct choice here.

What case is Der Sternenhimmel in, and why?

Der Sternenhimmel is in the nominative case, because it is the subject of the sentence.

Der Sternenhimmel = subject (who/what looks so lively?)
sieht = verb
heute so lebendig aus = predicate parts

Der is the masculine nominative singular definite article.
Sternenhimmel is a masculine noun: der Himmel, der Sternenhimmel.

What kind of word is Sternenhimmel, and why is it one word?

Sternenhimmel is a compound noun:

die Sterne (the stars) → Sternen- (genitive/attributive-like form, “of stars”)
der Himmel (the sky)

German very often joins nouns together into one long noun to express what English usually does with “of” or noun + noun:

Sternenhimmel = the sky of stars / starry sky
Other examples: Hauptbahnhof (main train station), Autobahn (car + road), Zahnarzt (tooth + doctor).

As a noun, it is capitalized: der Sternenhimmel.

What’s the difference between lange and lang here, and why use lange?

Both lang and lange can function as adverbs meaning for a long time, but:

lange is the more common colloquial adverb form in this sense.
lang is a bit more restricted or can appear in fixed expressions.

In wir wollen lange draußen bleiben, lange is an adverb of duration modifying bleiben:

lange bleiben = to stay for a long time

You might see lang more in combinations like:

den ganzen Tag lang (all day long)
stundenlang (for hours)

But lange as a free adverb is completely natural and typical here.

Why is draußen used, and not something like außen or im Freien?

draußen = outside (as an adverb, where you are).
außen = on the outside (surface) of something (opposite of innen).
im Freien = outdoors, literally “in the open air”.

In wir wollen lange draußen bleiben:

We are talking about staying outside vs going inside, so draußen is the natural choice.

Other usages:

Ich bin draußen. (I’m outside.)
Die Jacke ist außen nass. (The jacket is wet on the outside.)
Wir essen heute im Freien. (We’re eating outdoors today.)

You could say:

…, dass wir lange im Freien bleiben wollen.
→ perfectly correct but a bit more formal/neutral. Draußen sounds more everyday.

Where can heute go in this sentence? Is its current position fixed?

Heute is an adverb of time and is quite flexible. In the original clause:

Der Sternenhimmel sieht heute so lebendig aus.

Other natural positions:

Heute sieht der Sternenhimmel so lebendig aus. (emphasis on today)
Der Sternenhimmel sieht so lebendig aus heute. (possible, but heute at the end sounds more spoken/colloquial and can stress today).

General rule of thumb for neutral order in a simple main clause:

Time – Manner – Place (TMP)
→ Heute (time) often comes relatively early, but word order can be changed to highlight different elements, as long as the finite verb stays in second position.

Why is sieht in the second position in the first clause?

Because of the verb-second (V2) rule in German main clauses:

In a main clause (not introduced by a subordinating conjunction like dass, weil, etc.), the finite verb must be in position 2.

Count syntactic elements, not individual words:

Der Sternenhimmel → 1st element (subject)
sieht → 2nd element (finite verb → must be here)
heute so lebendig aus → rest of the clause

If you move something else to the first position, the verb still stays second:

Heute sieht der Sternenhimmel so lebendig aus.
So lebendig sieht der Sternenhimmel heute aus.

But you cannot move the verb away from position 2 in a normal main clause.

Why does the verb come at the end after dass, unlike in the first clause?

Because dass introduces a subordinate clause, and subordinate clauses follow a verb-final pattern.

Main clause: Verb-second (V2)
- Der Sternenhimmel sieht heute so lebendig aus.
Subordinate clause with dass: Verb at the end
- …, dass wir lange draußen bleiben wollen.

Inside that subordinate clause, wollen is the finite verb and therefore must be at the very end.
When there is a full verb (bleiben) plus a modal (wollen), the full verb comes right before the modal:

…, dass wir bleiben wollen.
…, dass wir lange draußen bleiben wollen.

This verb-final order is typical for clauses introduced by: dass, weil, wenn, obwohl, als, während, etc.