Learn Hardcore Polish: W dzień pracuję w biurze, a w nocy czytam książkę. - In the daytime I work in the office, and at night I read a book.

w dzień – literally in (the) day, but it works as an adverbial time phrase, similar to during the day.
- dzień is in the accusative (same form as nominative for this noun).
- This is a common, idiomatic way to talk about time.
w nocy – literally in (the) night, also meaning at night.
- noc in locative is nocy, so this is locative after w.
- This one is more clearly a preposition + locative time expression.

In everyday speech, both patterns are normal and you just learn them as fixed expressions:

w dzień – during the day
w nocy – at night

Could you also say w dniu instead of w dzień?

You can say w dniu, but:

w dniu is more formal and usually used with a specific date:
- W dniu 1 stycznia… – On the 1st of January…
w dzień is the natural, everyday way to say during the day in this general, habitual sense.

So in this sentence, W dzień pracuję w biurze is the normal, idiomatic version.
W dniu pracuję w biurze would sound odd if you are talking about your regular routine.

Why is it w dzień but w biurze? Why do the endings change: dzień vs biurze?

Because w can take different cases depending on meaning:

w dzień – time expression, meaning during the day
- dzień is in accusative, acting like a time adverbial.
w biurze – location, meaning in the office
- biurze is locative singular of biuro (office), because w
  - place takes the locative when it means inside / at a location.

So:

Time: w dzień, w sobotę, w poniedziałek (accusative used for time adverbials)
Place: w biurze, w domu, w szkole (locative after w for location)

What case is książkę, and why is it not książka?

książkę is accusative singular of książka (a book).

The verb czytać (to read) takes a direct object in the accusative.
Feminine nouns in -a normally change to -ę in the accusative:
- książka → książkę
- herbata → herbatę
- kawa → kawę

So czytam książkę literally is I read (a/the) book, with book in the accusative as the thing being read.

In English we say I read books in general; why is it czytam książkę (singular) here?

Polish allows both:

czytam książkę – I read a book (usually understood as I read a book at night – maybe the same one, maybe just one at a time).
czytam książki – I read books (plural, more explicitly general).

The Polish sentence is fine as a typical/habitual action:
W dzień pracuję w biurze, a w nocy czytam książkę.
You can imagine: during the day I work; at night I sit down with a (some) book.

If you really want the general idea:

…a w nocy czytam książki. – and at night I read books.

Why is there no word for a or the before biurze and książkę?

Polish has no articles (no a/an or the).
Context tells you whether you should translate as a or the in English.

So:

w biurze could be:
- in an office
- in the office
  depending on context.
czytam książkę could be:
- I read a book
- I read the book

Here, as a description of a general routine, English usually prefers I work in an office and I read a book.

Why is it a w nocy, not i w nocy or ale w nocy?

Polish has three basic conjunctions here:

i – and (simple addition, same direction)
a – and / but (contrast, or different subject, or change of scene)
ale – but (stronger contrast, opposition)

In this sentence:

W dzień pracuję w biurze, a w nocy czytam książkę.

a shows a contrast between day and night activities:

during the day → I work
but / whereas at night → I read

You could say i, but a sounds more natural, because it highlights the difference between what happens during the day and at night.

Why is there no ja (I) in the sentence?

Polish usually omits subject pronouns when they are obvious from the verb ending.

pracuję – 1st person singular (I work)
czytam – 1st person singular (I read)

Because these endings clearly show I, you do not need ja:

W dzień pracuję w biurze… is normally understood as I work in an office during the day…

You only add ja for emphasis or contrast, e.g.:

Ja w dzień pracuję w biurze, a ona w domu.
I work in an office during the day, and she at home.

Does the Polish present tense here mean right now or usually?

In this sentence, the present tense expresses a habit / routine:

pracuję – I work (as a general, repeated activity)
czytam – I read (regularly at night)

Polish present tense often covers what English would call present simple for habits:

Codziennie wstaję o siódmej. – I get up at seven every day.
W dzień pracuję w biurze, a w nocy czytam książkę. – During the day I work in an office, and at night I read a book.

Context (words like w dzień, w nocy) makes it clear it’s habitual, not right now.

Can I change the word order, for example: Pracuję w biurze w dzień, a w nocy czytam książkę?

Yes. Polish word order is fairly flexible, especially with adverbials (time, place).

These are all grammatically correct, with subtle emphasis differences:

W dzień pracuję w biurze, a w nocy czytam książkę.
Neutral; topic is time, then what you do.
Pracuję w biurze w dzień, a w nocy czytam książkę.
Slightly more focus on pracuję w biurze as your job, then when (w dzień).
W biurze pracuję w dzień, a w nocy czytam książkę.
Emphasis: It’s in an office that I work during the day…

For a beginner, the original order is the most natural and easy to imitate.

What aspect are pracuję and czytam? Could I use popracuję or przeczytam instead?

pracuję – imperfective of pracować
czytam – imperfective of czytać

Imperfective here is correct because we are describing regular, ongoing, habitual actions.

Perfective forms would be:

popracuję – I will work (for some time / get some work done, one occasion)
przeczytam (książkę) – I will read (the whole book, finish it)

If you said:

W dzień popracuję w biurze, a w nocy przeczytam książkę.

this sounds more like a plan for a particular day:
During the day I’ll get some work done in the office, and at night I’ll (finish) reading a book.
Not your general routine anymore.

Can I say Dniem pracuję w biurze, a nocą czytam książkę instead?

Yes, but it is more literary / stylistic.

dniem – instrumental of dzień
nocą – instrumental of noc

These instrumental forms used as adverbs mean:

dniem – by day / in the daytime
nocą – by night / at night

So:

Dniem pracuję w biurze, a nocą czytam książkę.
is perfectly correct, but sounds a bit more poetic or elevated than
W dzień pracuję w biurze, a w nocy czytam książkę.

How do you pronounce pracuję, biurze, and książkę?

Rough, simplified guide:

pracuję – prah-TSOO-yeh
- c as in ts (like cats)
- uję = uje, here sounding like OO-yeh
biurze – BYOO-zheh
- biu like English byoo in beauty
- rz is pronounced like ż, similar to the s in measure
książkę – roughly KSHONSH-keh
- ks often sounds like ks or close to ksh
- ią is a nasal vowel, a bit like yon in yonk, but nasalized
- ż again like the s in measure
- final ę before k is often pronounced close to e with a slight nasal quality

Precise Polish pronunciation takes practice, but this should help you get close enough to be understood.