Od ósmej do południa pracuję w biurze, a po południu odpoczywam w domu.

Because od in this time expression takes the genitive case.

The full idea is really od ósmej godziny = from the eighth hour / from eight o’clock, but godziny is usually omitted. Since godzina is feminine, the ordinal number also appears in the feminine form:

• ósma = eighth
• od ósmej = from eight o’clock

So ósmej is the correct case form after od.

Because different prepositions require different cases.

• do
  • genitive → do południa = until noon
• po
  • locative in this expression → po południu = in the afternoon / after noon

The base noun is południe = noon. Its forms change depending on grammar:

• południe
• do południa
• po południu

This is very common in Polish: the preposition controls the case.

It means from eight o’clock until noon.

In natural English, you would usually say from eight until noon or from eight in the morning until noon.

In Polish, od ósmej by itself could in theory mean from eight, but with do południa the meaning is clearly the morning period.

Yes. Literally, po południu means after noon.

But in normal usage, it usually corresponds to English in the afternoon.

So:

• po południu odpoczywam = I rest in the afternoon

It is one of those expressions where the literal meaning and the natural translation are slightly different.

Yes, południe can mean both noon and south.

Polish uses the same word for both ideas, just as some languages connect midday and the south because of the position of the sun.

In this sentence, the meaning is definitely noon, because it appears in time expressions:

• do południa = until noon
• po południu = in the afternoon

If the topic were geography, then południe could mean the south.

Because Polish often omits subject pronouns when they are clear from the verb ending.

• pracuję = I work
• odpoczywam = I rest

The ending already tells you the subject is I, so ja is unnecessary unless you want emphasis.

For example:

• Pracuję w biurze = I work in an office / at the office
• Ja pracuję w biurze = I work in an office with extra emphasis, perhaps contrasting with someone else

Both are 1st person singular present tense forms.

• pracuję = I work
• odpoczywam = I rest

They describe what I do.

In this kind of sentence, the present tense usually expresses a habit, routine, or general schedule, not only what is happening right this second.

So the sentence sounds like:

• From eight until noon I work in the office, and in the afternoon I rest at home.

That sounds like a routine.

Because w meaning in or at a place usually takes the locative case when there is no movement.

So:

• w biurze = in the office
• w domu = at home / in the house

You would use a different case if there were motion toward somewhere:

• Idę do biura = I am going to the office
• Jestem w biurze = I am in the office

Here the speaker is located in those places, not moving into them, so the locative is used.

Because a often links two clauses with a slight contrast or change of topic, while i is more like a simple and.

Here the sentence contrasts two parts of the day:

• from eight until noon → I work
• in the afternoon → I rest

So a is very natural:

• pracuję w biurze, a po południu odpoczywam w domu

It feels like and then / whereas / while in the afternoon.

Using i would sound more like a simple addition, and a is better for this kind of contrast.

Because in Polish, when two clauses are joined by a, a comma is normally used.

So this is standard punctuation:

• Od ósmej do południa pracuję w biurze, a po południu odpoczywam w domu.

The comma separates the two parts of the sentence clearly.

Yes. Polish word order is more flexible than English word order.

The sentence as given is very natural and neutral:

• Od ósmej do południa pracuję w biurze, a po południu odpoczywam w domu.

But you could also say:

• Pracuję w biurze od ósmej do południa, a po południu odpoczywam w domu.
• Po południu odpoczywam w domu, a od ósmej do południa pracuję w biurze.

The basic meaning stays the same, but the emphasis changes depending on what you place earlier.

Most naturally, it describes a usual routine or regular schedule.

Polish present tense often covers both:

• what is happening now
• what someone regularly does

In this sentence, because it talks about parts of the day, it strongly suggests a routine:

• From eight until noon I work in the office, and in the afternoon I rest at home.

If you wanted to make it very clearly about right now today, you would usually add extra context.

A few useful points:

• ó is pronounced like u
  • ósmej sounds roughly like oos-mey with ó = u
• rz in biurze sounds like the s in measure
• w in Polish sounds like English v
• ł sounds like English w
• ę in pracuję is often pronounced less strongly at the end of a word, so learners should not expect a full English-style vowel there

Very rough approximations:

• od ósmej ≈ ot OOS-mey
• do południa ≈ do po-WOOD-nya
• pracuję w biurze ≈ pra-TSOO-ye v byoo-zhe
• odpoczywam w domu ≈ ot-po-CHI-vam v DO-moo

These are only approximations, but they can help at the start.

Because in Polish, when telling time with ordinal numbers, the understood noun is godzina, and godzina is feminine.

So even if godzina is not spoken, it is still felt in the grammar:

• ósma godzina = the eighth hour
• od ósmej = from the eighth hour / from eight o’clock

That is why you do not use a masculine or neuter form here.

Yes, depending on what nuance you want.

For example:

• Od ósmej rano do południa pracuję w biurze.

This makes it extra explicit that it is eight in the morning.

But in your original sentence, do południa already makes that clear, so rano is not necessary.

The original version is perfectly natural.