Learn Hardcore Danish: Det er rigtig koldt i haven. - It is really cold in the garden.

Here Det is a dummy subject, like English “it” in “It’s cold outside.”
It does not refer specifically to haven (the garden); it just refers to the general situation or the weather. Danish often uses det this way in impersonal sentences: Det regner, Det sneer, Det er mørkt, etc.

Can I also say Der er rigtig koldt i haven, and what is the difference between Det er and Der er here?

You can say Der er rigtig koldt i haven, and it is grammatically correct.

Det er rigtig koldt i haven focuses a bit more on “the situation” being cold: “It is really cold in the garden.”
Der er rigtig koldt i haven is more like “There is (a state of) real coldness in the garden,” focusing slightly more on the place.

In everyday speech, both are very common and often interchangeable.

Why is it rigtig koldt and not rigtigt koldt? Which one is more correct?

Both rigtig koldt and rigtigt koldt are accepted in modern Danish.

Traditionally, adverbs often take -t, so rigtigt koldt is the older “textbook” adverb form.
In actual spoken Danish, people very often use rigtig as an intensifier before adjectives: rigtig koldt, rigtig godt, rigtig flot.

So Det er rigtig koldt i haven is very natural, especially in spoken and informal written Danish.

Could I use meget or virkelig instead of rigtig here, and would it sound natural?

Yes, both work and sound natural, but with slightly different nuances:

Det er meget koldt i haven. – It is very cold in the garden. Neutral and common.
Det er virkelig koldt i haven. – It is really / truly cold in the garden. A bit stronger, often with more personal emphasis or surprise.
Det er rigtig koldt i haven. – Very common, slightly more colloquial and emotional, like English “really cold.”

All three are fine in everyday use.

Why is the adjective koldt and not kold? How does adjective agreement work here?

In Danish, adjectives agree with the gender and number of the noun (or pronoun) they describe.

det is neuter singular, so the adjective takes -t:
- Det er koldt.
- Æblet er koldt. (æblet is neuter.)

Compare with a common-gender subject:

Vinen er kold. (common gender, no -t)

So koldt matches the neuter det.

How do you pronounce koldt, and what happens with the dt at the end?

In modern Danish, the d in -dt is basically silent, and the word has a kind of short, “cut‑off” ending.

A rough English approximation is something like “kolt” with a little glottal catch in the middle.
The dt spelling is historical; the pronunciation is much simpler than it looks.

You see the same pattern in words like godt, hvidt, blødt, where -dt is written but the d is not clearly pronounced.

Why is it i haven and not på haven? How do I know when to use i versus på?

i usually means “in, inside” a space, while på is more like “on” (a surface) or “at” for some locations and institutions.

i haven – in the garden, seen as a space you are inside.
på bordet – on the table (surface).
på arbejde, på universitetet – at work, at the university (fixed expressions).

A garden is normally treated as an area you are in, so Danish uses i haven, not på haven.

Why is it haven and not have? How do definite forms like this work?

have is a common‑gender noun: en have (a garden).
Danish usually forms the definite with a suffix:

en have → haven = the garden
et hus → huset = the house

So i haven means “in the garden.”

If you say i en have, that means “in a garden” (some unspecified garden).
i den have would mean “in that garden (there)”, with extra emphasis on which garden.

Can I move i haven to the beginning and say I haven er det rigtig koldt?

Yes. Danish allows you to front the place phrase for emphasis, but you must keep the verb in second position:

Det er rigtig koldt i haven. (neutral order)
I haven er det rigtig koldt. (emphasis on in the garden)

Both are correct. The second version highlights the location a bit more, for contrast like: I haven er det rigtig koldt, men inde i huset er det dejligt varmt.

If I say Det er koldt i haven without rigtig, how does that change the meaning?

Without rigtig, the sentence is more neutral:

Det er koldt i haven. – It is cold in the garden. (simple statement)
Det er rigtig koldt i haven. – It is really/very cold in the garden. (stronger, more emphatic)

So rigtig just intensifies the adjective koldt.

Does have always mean a flower garden, or can this sentence also be understood as “backyard” or “yard”?

have basically means “garden” (an area with grass, plants, flowers, trees).

In a Danish context, haven around a house is often what English speakers might call a yard or backyard, so context decides the best English translation.

For a typical house, haven could be translated as “the yard” / “the backyard.”
For a more decorative or public space, it would be “the garden.”

The Danish word itself does not distinguish front yard vs. backyard; that would be forhave (front garden) and baghave (back garden).

Is Det er rigtig koldt i haven more spoken or written style? Are there more formal alternatives?

The sentence is perfectly natural in both spoken and informal written Danish. The word rigtig as an intensifier is slightly colloquial, but very widely used, also in newspapers and online writing.

A slightly more neutral or formal version would be:

Det er meget koldt i haven.
Det er virkelig koldt i haven.

But Det er rigtig koldt i haven is not wrong or slangy; it is standard contemporary Danish.