Po południu zrobiło się zimno, więc wróciliśmy do domu.

Po południu means in the afternoon or literally after noon.

It uses the preposition po, which normally takes the locative case.
So południe changes to południu:

• południe = noon
• po południu = after noon / in the afternoon

This is a very common fixed time expression in Polish.

Examples:

• Rano = in the morning
• Po południu = in the afternoon
• Wieczorem = in the evening
• W nocy = at night

So the form po południu is not random; it is the correct case form after po.

Yes, literally it looks something like it made itself cold, but in real usage it means:

• it got cold
• it became cold

This is a very common Polish structure:

• zrobiło się ciemno = it got dark
• zrobiło się cicho = it became quiet
• zrobiło się późno = it got late
• zrobiło się zimno = it got cold

So zrobić się + adjective/adverb-like weather/state word often expresses a change of state.

Compare:

• Było zimno. = It was cold.
• Zrobiło się zimno. = It got cold / It became cold.

The second one emphasizes that the weather changed.

Because zimno here is not describing a noun like cold water or a cold day. It is part of an impersonal expression about the general situation or weather.

Compare:

• zimny dzień = a cold day
• zimna kawa = cold coffee
• Jest zimno. = It is cold.
• Zrobiło się zimno. = It got cold.

In sentences like jest zimno or zrobiło się zimno, Polish uses zimno, not zimny, because there is no noun being described.

English also does something similar:

• The day is cold → adjective describing day
• It is cold → general weather statement

Polish just uses a different form for that general statement.

In zrobiło się zimno, there is no real subject like the weather or the air. It is an impersonal construction.

In Polish, impersonal past-tense sentences of this kind usually use the neuter singular form:

• zrobiło się zimno
• ściemniło się
• udało się
• wydawało się

So zrobiło is in the default neuter singular because there is no specific noun controlling agreement.

You can think of it as the Polish equivalent of English it in weather/time expressions:

• It got cold
• It got dark

But Polish usually does not add a word for it here.

Więc means:

• so
• therefore
• thus

It does not mean because.

In this sentence:

• Po południu zrobiło się zimno, więc wróciliśmy do domu.
• It got cold in the afternoon, so we went back home.

So the logic is:

1. it got cold
2. therefore we returned home

Compare:

• bo = because
• więc = so / therefore

Examples:

• Wróciliśmy do domu, bo zrobiło się zimno. = We went back home because it got cold.
• Zrobiło się zimno, więc wróciliśmy do domu. = It got cold, so we went back home.

Because Polish verbs usually already show who the subject is.

Wróciliśmy means we returned / we went back. The ending -liśmy tells you it is we in the past tense.

So Polish often drops subject pronouns unless they are needed for emphasis, contrast, or clarity.

Compare:

• Wróciliśmy do domu. = We went back home.
• My wróciliśmy do domu. = We went back home. / We were the ones who went back home.

The version with my sounds more emphatic.

This is very normal in Polish:

• Idę = I am going
• Idziemy = we are going
• Wróciłem / wróciłam = I returned
• Wróciliśmy / wróciłyśmy = we returned

This is a question of aspect, which is very important in Polish.

• wrócić = perfective = to return, to come back, with focus on completion
• wracać = imperfective = to be returning / to return repeatedly / to be in the process of returning

Here, wróciliśmy is perfective past, meaning the return is seen as a completed event:

• we went back home
• we returned home

If you said wracaliśmy, it would mean something more like:

• we were going back
• we used to go back
• we were returning

That would not fit as well here, because the sentence describes a clear sequence:

1. it got cold
2. we returned home

So wróciliśmy is the natural choice.

Polish past tense is built directly into the verb form.
In wróciliśmy, several pieces are packed together:

• wróci- = the past stem
• -li- = past plural marker
• -śmy = we

So one word contains both the idea of the verb and the subject.

This is why Polish does not need an extra word like English did or always a separate pronoun.

A few examples:

• wróciłem = I returned (male speaker)
• wróciłam = I returned (female speaker)
• wróciliśmy = we returned (group with at least one male, or mixed group)
• wróciłyśmy = we returned (all-female group)

Yes. In the Polish past tense plural, the form can show gender.

• wróciliśmy = we returned for a masculine-personal group
  This means:

  • a group of men, or
  • a mixed group including at least one male

• wróciłyśmy = we returned for an all-female group

So wróciliśmy suggests that the speakers are either:

• all male, or
• mixed male/female

This is something English does not show, so it often surprises learners.

Because do means to / into in the sense of movement toward a place, and it requires the genitive case.

So:

• dom = house / home
• domu = genitive singular form

That gives:

• do domu = to home / to the house / homeward

Compare:

• Jesteśmy w domu. = We are at home.
  Here w means in/at, so there is no movement.
• Wracamy do domu. = We are returning home.
  Here there is movement toward home.

So:

• w domu = at home
• do domu = to home / homeward

In this sentence, do domu is best translated as home.

Even though dom literally means house, in many contexts Polish uses it the way English uses home.

So:

• wrócić do domu = to return home
• iść do domu = to go home
• być w domu = to be at home

If you translate it too literally as to the house, it can sound less natural in English unless the context specifically focuses on the building.

Because Polish punctuation normally separates clauses like these with a comma.

The sentence has two parts:

1. Po południu zrobiło się zimno
2. więc wróciliśmy do domu

Since więc introduces the result, a comma is standard:

• ..., więc ...

This is similar to English punctuation with so in many sentences:

• It got cold, so we went home.

Yes. Polish word order is flexible, although some versions sound more natural than others depending on emphasis.

The original sentence:

• Po południu zrobiło się zimno, więc wróciliśmy do domu.

Other possible versions:

• Zrobiło się zimno po południu, więc wróciliśmy do domu.
• Wróciliśmy do domu, bo po południu zrobiło się zimno.

The original order sounds very natural because it starts with the time expression po południu, setting the scene first.

Polish often moves parts around to change emphasis:

• what happened first in the sentence may be what the speaker wants to highlight
• the basic meaning can stay the same

Usually, in everyday Polish, po południu means in the afternoon.

Depending on context, it can also carry the more literal sense after noon, but most of the time learners should understand it as the standard time-of-day expression in the afternoon.

For example:

• Spotkamy się po południu. = We’ll meet in the afternoon.
• Po południu zaczęło padać. = It started raining in the afternoon.

So in normal conversation, think of it primarily as in the afternoon.

Było zimno is possible, but it means something slightly different.

• Było zimno = it was cold
• Zrobiło się zimno = it got cold / it became cold

So:

• Po południu było zimno means the weather was cold in the afternoon.
• Po południu zrobiło się zimno means it became cold in the afternoon, emphasizing a change.

Stało się zimno is not the normal phrase here.
Polish strongly prefers zrobiło się zimno for this kind of weather/state change.

So the most natural choice in this sentence is definitely:

• zrobiło się zimno