Learn Hardcore Czech: V parku je nový dům a můj pes rád spí v něm. - In the park there is a new house and my dog likes to sleep in it.

V parku je nový dům corresponds to English There is a new house in the park.
This is the usual Czech way to introduce the existence of something in a place (an existential sentence). The place (v parku) often comes first.
Nový dům je v parku corresponds more to The new house is in the park.
This sounds like you are talking about some specific house that is already known in the context and you are now saying where it is.

So the original word order is natural because we are probably introducing the information that in the park, there is a new house.

Why is it v parku and not v park?

The preposition v (in) normally requires the locative case when you talk about being in a place (location, not movement).

park is the basic (nominative) form of the noun.
In the locative singular, park → parku.

So, for static location:

v parku = in the park (locative)

If you talked about movement into the park, you would usually use another preposition:

do parku = into the park (genitive)

v park is not correct in modern standard Czech for this meaning.

What case is parku, and why does park change to parku?

Parku is locative singular.

Czech masculine inanimate nouns like park usually have:

Nominative singular (basic form): park
Locative singular: parku

The locative is used mainly after certain prepositions to express location, such as:

v parku – in the park
o parku – about the park
na mostě, v domě, na stole etc. (other examples with locative)

So parku is just the regular locative form of park required by v here.

Why is it nový dům, and not nové dům or nová dům?

Because dům is masculine inanimate, and the adjective must agree with the noun in gender, number, and case.

dům – masculine inanimate, singular, nominative
The correct masculine inanimate nominative singular form of nový is nový.

So:

nový dům = new house (correct)
nová dům – would match a feminine noun, so it’s wrong here
nové dům – could match a neuter noun in nominative singular or any gender in nominative plural, so also wrong here

Agreement pattern:

nový dům (m. inanimate)
nová kniha (feminine)
nové auto (neuter)

Is dům masculine or neuter, and how is it declined?

Dům is masculine inanimate.

Its singular forms (most common pattern) are roughly:

Nominative: dům (the house)
Genitive: domu (of the house)
Dative: domu (to the house)
Accusative: dům (I see the house)
Locative: (v) domě or sometimes (v) domu (in the house)
Instrumental: domem (with the house)

So in your sentence, dům is in nominative because it is the grammatical subject of je.

Why is it můj pes and not moje pes or mého pes?

The possessive pronoun must agree with the noun in gender, number, and case.

pes (dog) is masculine animate, nominative singular.
The correct nominative singular masculine form of my is můj.

So:

můj pes = my dog (subject form, nominative) – correct
moje pes – moje is typically used with feminine nouns in nominative singular or with all genders in plural, so it doesn’t match pes here.
mého pes – mého is a genitive/accusative form; nominative subject needs můj, not mého.

Other examples:

moje kočka (my cat – feminine)
moje auto (my car – neuter)

What exactly does rád spí mean, and why do we use rád instead of a verb like “to like”?

Rád spí literally means he gladly sleeps or he happily sleeps, but in real usage it means he likes to sleep / he enjoys sleeping.

In Czech, a very common way to say “X likes doing Y” is:

X rád dělá Y
- Můj pes rád spí. – My dog likes to sleep.
- Rád čtu. – I like reading.
- Ráda vařím. – I (female) like cooking.

Rád changes with gender and number:

rád – masculine singular (e.g. Můj pes rád spí.)
ráda – feminine singular (e.g. Moje sestra ráda čte.)
rádo – neuter singular
rádi – masculine personal plural (e.g. Rádi cestujeme.)

You can also say mít rád:

Můj pes má rád spaní. – My dog likes sleeping.
Mám rád svého psa. – I like my dog.

But rád spí is shorter and very natural for likes to sleep.

What does v něm refer to, and why is it něm?

V něm means in it / in him and here it refers back to dům (the house).

Grammatically:

The pronoun is on (he / it for masculine nouns).
Locative singular of on is něm.
With the preposition v (in), which requires locative, we get v něm = in it.

So:

V parku je nový dům a můj pes rád spí v něm.
= There is a new house in the park and my dog likes to sleep in it (in that house).

Could I say … a můj pes rád spí v domě instead of v něm?

Yes, … a můj pes rád spí v domě is grammatically correct and understandable:

v domě = in the house (locative of dům)

However, Czech often uses pronouns to avoid repeating the same noun right away. Using v něm sounds more natural and less repetitive in this short context.

So both are possible:

… a můj pes rád spí v domě. – OK, a bit repetitive.
… a můj pes rád spí v něm. – Very natural: … and my dog likes to sleep in it.

Why is it něm and not něj or něho?

These are different case forms of the same pronoun on (he/it):

něm – locative (used after v, o, na for location, etc.)
- v něm – in him/it
- o něm – about him/it
něj / něho – accusative (after many prepositions expressing direction, object, etc.)
- na něj / na něho – onto him/it
- pro něj / pro něho – for him

In your sentence, the preposition v expresses location (where the dog sleeps), so it needs locative → v něm, not v něj.

Why is it v něm, not ve něm, when sometimes I see ve instead of v?

Czech has two variants of this preposition: v and ve. Ve is used mainly for easier pronunciation, especially:

before words starting with v- or f- (e.g. ve Francii, ve Vídni)
before some difficult consonant clusters (e.g. ve škole)

Before něm, pronunciation is easy with v, so the normal form is:

v něm, not ve něm

So:

v něm – correct and natural
ve městě, ve škole, ve Francii – places where ve sounds better than v.

Can the word order můj pes rád spí v něm change? For example, is můj pes v něm rád spí possible?

Yes, Czech word order is relatively flexible, and all of these are grammatically possible:

můj pes rád spí v něm – neutral: my dog likes to sleep in it
můj pes v něm rád spí – emphasizes in it (as opposed to somewhere else)
v něm můj pes rád spí – strong focus on v něm at the beginning: it’s in that place that my dog likes to sleep

In everyday speech, the original … a můj pes rád spí v něm is the most neutral and typical. The other orders are used when you want to highlight or contrast something.