Ona gotuje kolację w kuchni, a ja w tym czasie czytam książkę.

That is true: Polish often omits subject pronouns because the verb ending usually shows who is doing the action.

So you could say:

Gotuje kolację w kuchni, a w tym czasie czytam książkę.

But in your sentence, ona and ja help create a clear contrast:

She is doing one thing, I am doing another.

So here the pronouns are not required for grammar, but they are natural for emphasis and contrast.

They are present-tense verb forms.

• gotuje = 3rd person singular of gotować
  → he/she/it cooks / is cooking
• czytam = 1st person singular of czytać
  → I read / I am reading

The endings tell you the subject:

• -ę / -am / -em etc. can show I
• -e / -i / -y etc. can show he/she/it

That is one reason Polish can leave out subject pronouns so often.

Because both words are direct objects, so they are in the accusative case.

Base forms:

• kolacja = dinner
• książka = book

Accusative singular:

• kolację
• książkę

A very common pattern in Polish is:

• feminine nouns ending in -a
• change to -ę in the singular accusative

So:

• gotować kolację
• czytać książkę

This is one of the first case patterns learners usually meet.

Because the preposition w meaning in usually requires the locative case when it describes location.

Base form:

• kuchnia = kitchen

After w:

• w kuchni = in the kitchen

So the ending changes because the noun is no longer in the dictionary form.

It is also the locative case, because it follows w.

Base forms:

• ten = this
• czas = time

After w:

• w tym czasie = during this time / meanwhile / at that time

So:

• tym is the locative form of ten
• czasie is the locative form of czas

This whole phrase is very natural in Polish and often means something like meanwhile.

A often translates as and, but it is not exactly the same as i.

In this sentence, a connects two clauses while also showing a contrast or switch of topic:

• She is cooking dinner,
• and meanwhile I am reading a book.

So a is perfect here because it feels like:

• and
• while
• whereas
• meanwhile

depending on context.

By contrast, i is more of a plain, simple and.

In Polish, the present tense can cover both ideas:

• I read
• I am reading

So:

• gotuje can mean cooks or is cooking
• czytam can mean read or am reading

In this sentence, because of the context, the natural interpretation is ongoing actions happening now:

• she is cooking
• I am reading

Polish does not have a separate tense form like English am reading.

Both verbs are imperfective:

• gotować
• czytać

That matters because imperfective verbs are used for:

• actions in progress
• repeated actions
• habitual actions
• unfinished actions

That fits this sentence well, because both actions are happening at the same time.

A useful contrast:

• gotować = to cook, be cooking

• ugotować = to cook something completely, finish cooking

• czytać = to read, be reading
• przeczytać = to read through, finish reading

Also, in Polish, perfective verbs do not normally have a true present meaning. Their present forms usually refer to the future.

Polish has no articles like English a/an/the.

So Polish simply says:

• kolację
• książkę
• kuchni

without a separate word for a or the.

The listener understands whether something is definite or indefinite from:

• context
• word order
• stress
• the situation

That is why English speakers often feel something is missing at first, but in Polish this is completely normal.

Yes. Polish word order is more flexible than English because case endings show the grammatical roles.

Your sentence is a very natural neutral version:

Ona gotuje kolację w kuchni, a ja w tym czasie czytam książkę.

But other orders are possible, for example:

• W kuchni ona gotuje kolację, a ja w tym czasie czytam książkę.
• Ona w kuchni gotuje kolację, a ja czytam książkę w tym czasie.

These versions may sound more marked or place emphasis on different parts.

So the meaning stays similar, but the focus changes.

Yes, normally yes.

In Polish, when a joins two clauses with their own verbs, a comma is standard:

• Ona gotuje..., a ja czytam...

This is very similar to Polish comma rules with other clause-linking conjunctions.

So in your sentence, the comma is correct and expected.

Yes.

You could say:

Ona gotuje kolację w kuchni, a ja czytam książkę.

That is still grammatically correct.

Adding w tym czasie makes the time relationship more explicit. It emphasizes that while she is doing that, I am doing this.

So the phrase is not required, but it adds a nice sense of simultaneous action.

Because a ja is a very natural contrast pattern in Polish.

It highlights the switch from one subject to another:

• ona does one thing,
• a ja do another.

This structure is extremely common in Polish when comparing or contrasting people, actions, or situations.

For example:

• On pracuje, a ja odpoczywam.
• Ona pije herbatę, a ja kawę.

So a ja here sounds very normal and idiomatic.