Termostaten er på væggen.

In Danish, the definite article the is usually added to the end of the noun instead of being a separate word before it.

• en termostat = a thermostat

• termostaten = the thermostat

• en væg = a wall
• væggen = the wall

So Termostaten er på væggen literally matches The thermostat is on the wall.

Both termostat and væg are common gender nouns, and common gender nouns usually take -en in the definite singular.

Because Danish normally uses a suffix for the definite form.

English:

• the thermostat
• the wall

Danish:

• termostaten
• væggen

This is one of the biggest differences from English. Instead of putting a word before the noun, Danish often attaches the definiteness directly to the noun.

A separate definite word does exist in Danish, but it is mainly used when there is an adjective before the noun:

• væggen = the wall
• den hvide væg = the white wall

So in your sentence, no separate the is needed.

Er means is.

It is the present tense of at være, which means to be.

Examples:

• jeg er = I am
• du er = you are
• han er = he is
• termostaten er = the thermostat is

So the structure is very close to English:

• Termostaten = The thermostat
• er = is
• på væggen = on the wall

På usually means on, and that is the natural choice here because a thermostat is attached to the surface of a wall.

• på væggen = on the wall

For an English speaker, this is straightforward in this sentence. Danish uses på in many places where English also uses on.

However, Danish prepositions do not always match English perfectly in every expression, so it is good to learn them phrase by phrase.

Here, på væggen is exactly what you would expect.

The basic noun is væg. When the definite ending -en is added, the spelling becomes væggen.

So:

• væg = wall
• væggen = the wall

This doubling is just part of the standard spelling form. You should learn væggen as the normal definite singular form.

Yes, gender matters in Danish because it affects articles and endings.

Both nouns here are common gender:

• en termostat
• en væg

Because they are common gender, they take:

• en in the indefinite singular
• -en in the definite singular

Compare that with a neuter noun:

• et hus = a house
• huset = the house

So for this sentence:

• termostat → termostaten
• væg → væggen

Yes, grammatically you could, but the meaning changes.

• Termostaten er på væggen = The thermostat is on the wall
• En termostat er på væggen = A thermostat is on the wall

The second sentence sounds less natural in many normal situations unless you are introducing it for the first time or speaking more generally.

For example:

• Der er en termostat på væggen. = There is a thermostat on the wall.

That version often sounds more natural than En termostat er på væggen if you are simply pointing out that one exists.

Yes, in this basic statement the word order is very similar to English:

• Termostaten = subject
• er = verb
• på væggen = prepositional phrase

So the pattern is:

Subject + verb + place

That said, Danish word order can change if you move another element to the front for emphasis. For example:

• På væggen er termostaten.

This is grammatical, but much less neutral. The ordinary, natural sentence is:

• Termostaten er på væggen.

A rough English-friendly guide would be something like:

ter-mo-STA-ten er paw VEL-yen

But Danish pronunciation varies by region, and a rough spelling can only help a little.

A few useful notes:

• termostaten has the stress on sta
• på sounds somewhat like paw
• væggen does not sound exactly like it looks to an English speaker

If you are learning pronunciation, it is best to hear the sentence spoken by a native speaker and repeat it, because Danish vowels and soft consonants are hard to guess from spelling alone.

Yes, and that is actually very natural Danish.

• Termostaten er på væggen = The thermostat is on the wall
• Termostaten sidder på væggen = literally The thermostat sits on the wall

In Danish, sidder is often used for things that are attached or positioned on a surface. So for objects mounted on a wall, sidder can sound very natural.

The version with er is still correct and simpler for learners, but sidder is a useful word to know.

No. In modern Danish, nouns are not capitalized unless they begin a sentence or are part of a proper name.

So:

• Termostaten er på væggen.

Only Termostaten has a capital letter here because it is the first word of the sentence, not because it is a noun.