Learn Hardcore Danish: Lyden er lav i stuen. - The sound is low in the living room.

So Lyden er lav i stuen literally means The sound is low in the living room.
If you said Lyd er lav i stuen, it would sound wrong to a Dane; you need the definite form lyden here.

What gender is lyd, and how do I use it in other forms?

Lyd is a common gender noun (n-Word).

Key forms:

Indefinite singular: en lyd (a sound)
Definite singular: lyden (the sound)
Indefinite plural: lyde (sounds)
Definite plural: lydene (the sounds)

Examples:

Jeg hører en mærkelig lyd. – I hear a strange sound.
Lydene udenfor er meget høje. – The sounds outside are very loud.

Can lav really mean “quiet”? I thought lav meant “low” like in height.

Yes, lav means low, and that includes:

Physical height:
- Et lavt bord – a low table
Level/intensity, including sound volume:
- Lyden er lav. – The sound is low (quiet)
- Tal lidt lavere. – Speak a bit more quietly / at a lower volume.

So in this sentence, lav refers to the volume level of the sound, not its physical position.

Could I say Lyden er stille i stuen instead of Lyden er lav i stuen?

Not in the same way.

lav with lyd is about volume level (quiet vs loud).
stille means quiet/silent as a general atmosphere, not as a technical “low volume”.

Natural uses:

Lyden er lav i stuen. – The sound volume is low in the living room.
Der er stille i stuen. – It is quiet in the living room (overall, not much noise).

Lyden er stille is not idiomatic Danish; you would normally say lyden er lav or lyden er svag for a weak/quiet sound.

What is the difference between lyd and støj?

lyd = sound (neutral word; any sound)
støj = noise (usually unpleasant or unwanted sound)

Compare:

Lyden er lav i stuen. – The sound is low in the living room.
Der er meget støj i stuen. – There is a lot of noise in the living room.

You wouldn’t normally use støj with lav in this way; you’d say something like Der er ikke så meget støj (there isn’t so much noise) instead.

Why is it i stuen and not something like på stuen?

For rooms inside a house or building, Danish normally uses i (in):

i stuen – in the living room
i køkkenet – in the kitchen
i soveværelset – in the bedroom

på is used with some specific places (e.g. på arbejde, på skolen, på hospitalet), but not with normal rooms in a home. So på stuen would be wrong in standard Danish in this context.

Why is it i stuen and not i stue?

Same reason as with lyd: you need the definite form for a specific room.

en stue – a living room
stuen – the living room

You’re talking about a particular living room (probably the one in the house you’re in), so you say i stuen.

i stue would sound incomplete/incorrect, just like in living room is wrong in English.

Could I say I stuen er lyden lav instead? Is that still correct?

Yes, I stuen er lyden lav is also correct.

Lyden er lav i stuen. – neutral word order, subject first.
I stuen er lyden lav. – puts extra emphasis on the location i stuen (“In the living room, the sound is low”).

Both are grammatical; the difference is a matter of emphasis and style.

Where does the adjective lav have to go in a Danish sentence like this?

With the verb at være (to be), the basic pattern is:

[subject] + er + [adjective] + [place]

So:

Lyden er lav i stuen.

Putting lav after i stuen – Lyden er i stuen lav – is not natural in Danish. The normal position for the adjective is directly after the form of “er” and before the place expression.

How would I make this plural, if I wanted to talk about sounds in several rooms?

You change both the noun and possibly the place phrase, depending on what you mean.

Several sounds in one living room:
- Lydene er lave i stuen.
  – The sounds are low in the living room.
The sound in several living rooms:
- Lyden er lav i stuerne.
  – The sound is low in the living rooms.
Several sounds in several living rooms:
- Lydene er lave i stuerne.
  – The sounds are low in the living rooms.

Notice the plural forms: lydene, lave, stuerne.

How do you pronounce lyden, lav, and stuen?

Approximate pronunciations (in simple English terms):

lyden – roughly “LEW-then”
- Danish y is like the French u in lune or German ü in müde.
lav – often like “laow” (a bit between lahv and English loud but shorter)
stuen – roughly “STOO-en” (with a short, unstressed -en)

Real Danish pronunciation has some subtleties (vowel length, soft d in lyden, etc.), but these approximations are close enough to be understood.

Could I say Lyden er meget lav i stuen to mean “The sound is very low in the living room”?

Yes. That’s perfectly natural.

Lyden er lav i stuen. – The sound is low in the living room.
Lyden er meget lav i stuen. – The sound is very low in the living room.

You can also use other degree words:

Lyden er lidt lav i stuen. – The sound is a bit low in the living room.
Lyden er for lav i stuen. – The sound is too low in the living room.