W domu nie marzniemy, bo używamy dużego koca.

Because Polish prepositions control specific cases, and w with a static location (being in/at somewhere, not movement into somewhere) takes the locative case.

• The noun dom (house/home) in the locative singular is domu.
• So the pattern is:
  
  • w + locative → w domu = in/at home
• *w dom is wrong because dom is nominative/accusative, not locative.
• w domie would be possible only if the noun’s locative were domie, but for dom the standard form is domu, not domie.

So w domu is simply “in/at the house / at home” in correct locative case.

w domu can mean both, and context decides which English translation feels more natural.

• at home (general, where you live):
  
  • W domu nie marzniemy = We don’t feel cold at home.
• in the house (contrasting inside vs outside):
  
  • Na dworze jest zimno, ale w domu nie marzniemy
    Outside it’s cold, but in the house we don’t feel cold.

Polish usually doesn’t force this distinction; w domu covers both ideas.

In Polish, the standard way to negate a verb is to put nie directly before it:

• nie marzniemy = we are not freezing / we don’t feel cold

Putting nie after the verb (*marzniemy nie) is ungrammatical in normal sentences.

So the pattern is:

• nie + verb → nie marznę, nie marzniesz, nie marzniemy, etc.

You can move other words around, but nie stays directly in front of the verb it negates.

marzniemy is the 1st person plural present form of marznąć.

• marznąć = to be (getting) cold, to freeze, to feel cold (a process or state)
• marzniemy = we are (getting) cold / we feel cold / we’re freezing (now, in general, or habitually)

Comparison:

• Nie marzniemy
  We don’t feel cold / we aren’t freezing (neutral way to say it).
• Nie jest nam zimno
  Literally: It is not cold to us → also we’re not cold.
  Slightly more “impersonal” construction, very natural in Polish.
• Zmarzliśmy
  Past perfective: We (have) got very cold / we froze (earlier) — a completed event.

So in your sentence:

• W domu nie marzniemy = At home we don’t feel cold / we don’t freeze (there).
  It describes a general ongoing/habitual situation, not a one-time past event.

Present tense of marznąć (imperfective):

• ja marznę – I (am) freezing / I feel cold
• ty marzniesz – you (sg) are freezing
• on / ona / ono marznie – he / she / it is freezing
• my marzniemy – we are freezing
• wy marzniecie – you (pl) are freezing
• oni / one marzną – they are freezing

Your sentence uses my marzniemy → (my) nie marzniemy.

bo and ponieważ both mean because.

In this sentence:

• bo używamy dużego koca = because we use a big blanket

You could also say:

• W domu nie marzniemy, ponieważ używamy dużego koca.

Differences:

• bo – very common, neutral, slightly more conversational.
• ponieważ – a bit more formal/bookish, often used in written or careful speech.

The comma before bo/ponieważ is standard in Polish when they introduce a clause:

• …, bo …
• …, ponieważ …

So the comma in W domu nie marzniemy, bo używamy dużego koca is correct and required.

Polish has aspect: imperfective vs perfective verbs.

• używać (imperfective) – to use (in general, habitually, or as an ongoing action)
  
  • używamy – we use / we are using
• użyć (perfective) – to use (once, as a single completed act)
  
  • użyjemy – we will use (once / at some point)

In your sentence we’re talking about a general, regular situation at home:

• bo używamy dużego koca
  because we use a big blanket (as a rule / regularly)

So the imperfective używamy is correct.
Użyjemy dużego koca would sound more like we will (at some point) use a big blanket (once).

Because the verb używać requires its object in the genitive case, not the accusative.

• Dictionary form: używać + GEN – to use something

The noun:

• koc (blanket) – nominative singular
• koca – genitive singular

The adjective duży must agree with the noun in case, number, and gender, so in genitive masculine singular it becomes dużego.

Hence:

• używamy dużego koca = we use a big blanket
  (literally: “we-use of-a-big blanket”)

If you used a verb that takes accusative, you would see the difference:

• Mamy duży koc. – We have a big blanket. (accusative: duży koc)
• Używamy dużego koca. – We use a big blanket. (genitive: dużego koca)

Here is a simplified singular declension (most relevant forms) for koc (masculine inanimate) and duży (masculine adjective):

Noun: koc (blanket)

• Nominative: koc – Ten koc jest duży. (This blanket is big.)
• Genitive: koca – Nie mam koca. (I don’t have a blanket.)
• Dative: kocowi – Przyglądam się kocowi.
• Accusative: koc – Widzę koc.
• Instrumental: kocem – Przykrywam się kocem.
• Locative: kocu – Leżę na kocu.
• Vocative: koc-u (rarely used in practice)

Adjective: duży (big) – masculine, singular

• Nominative: duży koc
• Genitive: dużego koca
• Dative: dużemu kocowi
• Accusative: duży koc (for inanimate masculine)
• Instrumental: dużym kocem
• Locative: dużym kocu

Your phrase dużego koca is genitive singular for both words, matching the verb używać, which takes genitive.

Polish word order is relatively flexible, so both are possible:

1. W domu nie marzniemy, bo używamy dużego koca.
2. Nie marzniemy w domu, bo używamy dużego koca.

Both are grammatical and natural. The difference is subtle:

• W domu nie marzniemy…
  Slightly more emphasis on at home as the setting.
• Nie marzniemy w domu…
  Slightly more emphasis on the fact we don’t feel cold (and specifying where later).

In everyday speech, both variants would be understood the same way.

marzniemy is pronounced roughly as:

• [mar-ZNYE-my] in a simplified phonetic transcription.

Details:

• rz is pronounced like ż / sz in vision → a voiced “zh” sound.
• marz- = “marzh” (like marzh in “marzh-neeemy”).
• -nie- = [ɲɛ] – like Spanish ñ in ni-e, a soft “nye”.
• Stress in Polish is almost always on the second-to-last syllable:
  mar-ZNIE-my.

So you get something close to: mar-ZNYE-my (with the stress on ZNIE).