Learn Hardcore Norwegian: Det er uvanlig stille i huset så tidlig, så jeg gjør yoga i stuen. - It is unusually quiet in the house this early, so I do yoga in the living room.

Det here is a dummy subject (sometimes called an expletive det). It doesn’t refer to anything specific; it just fills the subject position.
Norwegian usually wants an explicit subject, so Det er uvanlig stille i huset is normal and natural.

You could say Uvanlig stille i huset så tidlig in very informal, almost “note-like” or poetic style, but as a normal sentence in everyday speech or writing, you use Det er.

What are uvanlig and stille grammatically? Why are they together like that?

uvanlig = unusually / uncommon(ly)
It is an adjective meaning unusual, but here it functions like an adverb of degree (describing how quiet it is), similar to English unusually.
stille = quiet
Here it is an adjective describing the situation in the house.

So uvanlig stille literally means unusually quiet.
The pattern adverb (or adverb-like word) + adjective is very common in Norwegian:

veldig stille – very quiet
helt stille – completely quiet
ganske stille – quite quiet

uvanlig stille fits that same pattern.

Could I say veldig stille instead of uvanlig stille? Is there a difference?

Yes, you can say Det er veldig stille i huset. Both are correct, but the nuance is slightly different:

veldig stille = very quiet
→ Focus on the degree of quiet: it’s just very quiet.
uvanlig stille = unusually quiet
→ Implies this level of quiet is not normal for this house at this time; something is different from the usual situation.

In the original sentence, uvanlig stille suggests: it’s quieter than it normally is this early.

Why is it i huset and not på huset?

The choice between i and på is mostly idiomatic in Norwegian, but here:

i huset = in(side) the house
You are talking about what it is like inside the building.
på huset literally = on the house (like on the roof or the outside of the building), and would sound wrong in this context.

For locations inside enclosed spaces, Norwegian usually uses i:

i huset – in the house
i bilen – in the car
i stuen – in the living room

What exactly does så tidlig mean here, and could it go in a different place in the sentence?

så tidlig literally means so early. In this context it is like English this early or at this early hour.

Word order is quite flexible. Some possible positions:

Det er uvanlig stille i huset så tidlig, ... (original)
Så tidlig er det uvanlig stille i huset, ... (marked/emphatic: This early, it’s unusually quiet in the house)
Det er så tidlig uvanlig stille i huset, ... (possible but sounds less natural)

The original order i huset så tidlig is very natural: place + time.

The word så appears twice. Are they the same word and meaning in both places?

No, they are two different uses of så:

så tidlig
- Here så is an adverb of degree, meaning so (early).
..., så jeg gjør yoga i stuen.
- Here så is a conjunction, meaning so / therefore, linking cause and result:
  - It is unusually quiet in the house this early, *so I do yoga in the living room.*

They are spelled the same but function differently in the sentence.

Why is there a comma before så jeg gjør yoga i stuen?

In Norwegian, så used as a conjunction meaning so / therefore often introduces a new main clause. You usually put a comma between the two main clauses:

Det er uvanlig stille i huset så tidlig, så jeg gjør yoga i stuen.

Structure:

Det er uvanlig stille i huset så tidlig → main clause (reason)
så jeg gjør yoga i stuen → main clause (result), introduced by så

Two main clauses joined by a conjunction like og, men, for, så are normally separated by a comma in written Norwegian.

After så, why is the verb still in second position: så jeg gjør yoga and not så jeg yoga gjør?

Norwegian follows the V2 rule (verb-second word order) in main clauses:

The finite verb (here gjør) should be the second element in the clause.

In så jeg gjør yoga i stuen:

jeg = subject (first element)
gjør = finite verb (second element)
yoga i stuen = the rest

So jeg gjør yoga i stuen obeys V2.
You cannot say så jeg yoga gjør; that would break the normal word order and sound wrong in standard Norwegian.

Is jeg gjør yoga the most natural way to say I do yoga in Norwegian? Are there other options like jeg trener yoga or jeg driver med yoga?

Jeg gjør yoga is understandable and quite common, especially influenced by English do yoga, and many Norwegians say it.

Other options:

Jeg driver med yoga.
- Very idiomatic and natural. Often used about activities or hobbies you “do” regularly.
Jeg trener yoga.
- Also heard, especially where yoga is viewed as training/exercise.
Jeg holder på med yoga.
- Focuses on what you are in the middle of doing right now.

In the given sentence, any of these could work, but:

For a regular habit: Jeg driver med yoga i stuen.
For what you’re doing at the moment: Jeg gjør yoga i stuen / Jeg holder på med yoga i stuen.

How is stuen formed from stue, and what is the difference between stue, stuen, and stua?

stue = a living room (indefinite singular)
stuen = the living room (definite singular, more formal/standard Bokmål)
stua = the living room (definite singular, more colloquial form, also standard in some Bokmål variants and dialects)

So in the sentence:

i stuen = in the living room (a specific, known living room).

You would not normally say just i stue; you either mean a living room (i en stue) or the living room (i stuen / i stua).

Why is the present tense gjør used when this sounds like something I’m doing right now? Norwegian has no form like am doing?

Norwegian does not have a separate progressive tense like English am doing / is doing. The simple present is used for both:

Jeg gjør yoga.
= I do yoga or I am doing yoga, depending on context.

So:

To talk about a habit:
Jeg gjør yoga hver morgen. – I do yoga every morning.
To talk about right now:
(Phone rings) Kan jeg ringe deg senere? Jeg gjør yoga. – Can I call you later? I’m doing yoga.

In your sentence, context (it’s unusually quiet right now) makes jeg gjør yoga naturally understood as I’m doing yoga (now).

Are there any important pronunciation tips for this sentence, like silent letters or reduced forms?

Yes, a few useful points (standard East Norwegian):

Det
- Often pronounced like de [de], with a very weak or silent t.
er
- Short vowel [æ] or [e], often quite reduced in fast speech.
uvanlig
- Stress on the first syllable: U-van-lig. The g at the end is often weak or almost silent.
stille
- Double l gives a longer l sound; final e is a weak [ə]-sound.
huset
- hus [hʉːs]; -et is usually just a weak vowel [ə], the t is not clearly pronounced: [ˈhʉːsə].
stuen
- Typically [ˈstʉːən] or more merged [ˈstʉːn]; again, weak ending.

In normal, connected speech, the whole sentence will sound quite compressed, with reduced vowels especially in det, er, i, så, jeg.