Learn Hardcore Polish: W nocy ognisko ogrzewa nas, kiedy marzniemy. - At night the campfire warms us when we are freezing.

Using w noc would be incorrect in standard Polish in this meaning. If you wanted to say something like one night as a direct object, you might use noc in another context, but with w in a time expression you need the locative, so w nocy.

Could I also say nocą instead of w nocy?

Yes. Nocą is an adverbial form meaning at night / during the night. It is very natural and common.

So you can say:

W nocy ognisko ogrzewa nas, kiedy marzniemy.
Nocą ognisko ogrzewa nas, kiedy marzniemy.

Both are correct and sound natural. W nocy is a prepositional phrase (preposition + noun in locative), while nocą is a single adverbial form, but in practice they often mean the same thing in such sentences.

Why is the word order W nocy ognisko ogrzewa nas and not Ognisko ogrzewa nas w nocy?

Polish word order is flexible. Both of these are grammatically correct:

W nocy ognisko ogrzewa nas.
Ognisko ogrzewa nas w nocy.

The difference is more about emphasis:

W nocy ognisko ogrzewa nas… – puts w nocy at the beginning, so the time frame (at night) is the starting point, the “scene”.
Ognisko ogrzewa nas w nocy… – starts with ognisko, so the subject (the campfire) is the first focus.

In neutral, everyday speech, both orders are fine. Polish tends to place new or important information later in the sentence, and things that are already known or setting/background (like w nocy) often come first, but this is a tendency, not a strict rule.

Why is it ogrzewa and not just grzeje?

Both verbs are related, but they are a bit different:

grzać → grzeje = to warm, to heat (more general)
ogrzewać → ogrzewa = to warm up, to heat thoroughly, to provide warmth (often a bit more “complete” or “covering”)

In this sentence:

Ognisko ogrzewa nas – the campfire warms us up / keeps us warm.

If you said:

Ognisko grzeje nas,

it would still be understandable and basically correct, but ogrzewa sounds a bit more natural here when talking about an ongoing, comforting warmth from a fire on a cold night.

Also note the aspect:

ogrzewać – imperfective (ongoing, repeated action)
ogrzać – perfective (to warm up completely, as a single result)

Here we use ogrzewa (imperfective present) to talk about something that happens regularly or over a period of time.

What grammatical case is nas, and why is this form used?

Nas here is the direct object of the verb ogrzewa.

Grammatically, nas is:

1st person plural (we)
Accusative case (us)

In Polish, many pronouns use the same form for accusative and genitive, and nas is one of them. So:

Nominative: my (we)
Accusative: nas (us)
Genitive: nas (of us)

In this sentence, ognisko ogrzewa nas = the campfire warms us, so nas is in the accusative as the thing being warmed.

Can I change the position of nas, for example to Ognisko nas ogrzewa?

Yes. Pronouns like nas, mnie, cię can move around in the sentence. All of these are grammatically correct:

Ognisko ogrzewa nas.
Ognisko nas ogrzewa.

The difference is nuance:

Ognisko ogrzewa nas. – more neutral word order.
Ognisko nas ogrzewa. – often puts a bit more emphasis on nas (who is being warmed, as opposed to someone else).

In practice, both are common. With short pronouns, Polish speakers often put them just before the verb, but both options sound natural.

Why is there a comma before kiedy marzniemy?

In Polish, subordinate clauses (introduced by words like kiedy, że, bo, chociaż, etc.) are usually separated by a comma from the main clause.

Here:

Main clause: W nocy ognisko ogrzewa nas
Subordinate clause: kiedy marzniemy

Because kiedy marzniemy depends on the main clause (it tells us under what condition or when the main action happens), Polish punctuation rules require a comma:

W nocy ognisko ogrzewa nas, kiedy marzniemy.

Even if you swap the order:

Kiedy marzniemy, w nocy ognisko ogrzewa nas.

you still need the comma. So, unlike in English, you must keep the comma around such kiedy-clauses in Polish.

How should I understand kiedy marzniemy in terms of tense and meaning?

Literally, kiedy marzniemy is:

kiedy = when
marznąć (imperfective) → marzniemy = we are freezing / we are getting cold / we feel very cold

Both verbs in the sentence are in the present tense:

ogrzewa – warms
marzniemy – (we) are freezing / get cold

Together they describe either:

A habitual situation:
- At night the campfire warms us when we get cold.
Or a general, timeless statement about what happens under certain conditions.

Polish often uses the present tense for these general truths or repeated actions, just like English. The imperfective aspect (ogrzewa, marzniemy) emphasizes ongoing or repeated processes, not a single completed event.

Could I say kiedy jest nam zimno instead of kiedy marzniemy? Is there a difference?

Yes, you could say:

W nocy ognisko ogrzewa nas, kiedy jest nam zimno.

Both options are natural, but the nuance is slightly different:

kiedy marzniemy – literally when we (start to) freeze, more vivid, suggests we are really cold and perhaps getting colder.
kiedy jest nam zimno – literally when it is cold to us, more neutral, simply when we feel cold.

So:

marznąć is stronger, more about freezing / getting very cold.
jest nam zimno is milder, more about feeling cold in general.

Which you choose depends on how intense you want the cold to feel in the sentence.

What is the difference between ognisko and ogień?

Both relate to fire, but they are used differently:

ognisko – usually means a campfire / bonfire (a specific, controlled fire you sit around). It is a neuter noun.
- W nocy ognisko ogrzewa nas. = At night the campfire warms us.
ogień – means fire in a more general sense (the element, flames, a blaze, also metaphorical).
- Ogień jest gorący. = Fire is hot.
- Wybuchł ogień w lesie. = A fire broke out in the forest.

In this sentence, ognisko is chosen because we are clearly talking about a campfire that people sit around to get warm.

How is rz pronounced in words like ogrzewa and marzniemy, and why is it written that way?

In modern standard Polish, rz is usually pronounced the same as ż:

rz ≈ ż = a sound similar to s in measure or vision.

So:

ogrzewa is pronounced approximately like og-żewa.
marzniemy has rz pronounced like ż, but then it’s followed by another consonant, so you hear a cluster.

Historically, rz developed from an r followed by a palatal sound, so it kept a special spelling. Today, the spelling rz is mostly historical/etymological; you usually just need to memorize where to write rz and where to write ż, even though they sound the same in most positions.