Ta książka jest ciekawa, często o niej myślę.

Because książka is a feminine singular noun in the nominative case here, and ta is the nominative feminine singular form of this.

• ten = masculine nominative singular
• ta = feminine nominative singular
• to = neuter nominative singular

Since książka is feminine, Polish uses ta.

You would see tę książkę if the noun were in the accusative, for example:

• Widzę tę książkę. = I see this book.

But in your sentence, książka is the subject, so nominative is needed:

• Ta książka jest ciekawa.

Because adjectives in Polish must agree with the noun they describe in gender, number, and case.

Here, książka is:

• feminine
• singular
• nominative

So the adjective must match:

• ciekawy = masculine
• ciekawa = feminine
• ciekawe = neuter / some plural uses

That is why we get:

• Ta książka jest ciekawa.

If the noun were masculine, you would say:

• Ten film jest ciekawy.

Because it is the subject of the clause.

In Ta książka jest ciekawa, the book is the thing being talked about: it is the book that is interesting. Subjects are normally in the nominative case in Polish.

So:

• książka = nominative, because it is the subject

Compare:

• Ta książka jest ciekawa. = nominative subject
• Czytam tę książkę. = accusative object

Because the preposition o in the meaning about requires the locative case.

The pronoun ona changes form after a preposition and in different cases:

• ona = she
• nią = her (instrumental, or accusative after some prepositions)
• niej = her / it in forms used after certain prepositions, including o

So:

• myślę o niej = I think about her/it

You cannot say o ona, because ona is the basic nominative form, and the preposition o does not take nominative here.

Because Polish pronouns follow the grammatical gender of the noun, not whether something is a person.

Książka is a feminine noun, so when you refer back to it, you use the same feminine pronoun forms that are also used for she/her.

So in this sentence:

• o niej refers to książka
• in English, we translate it as about it
• literally, the Polish form is the same one used for feminine reference

This is very common in Polish. A non-human thing can still take a masculine, feminine, or neuter pronoun depending on the noun’s grammatical gender.

Because in Polish, the verb ending usually already shows who the subject is.

Myślę means:

• I think

The ending -ę tells you it is 1st person singular, so ja is not necessary.

That is why Polish often drops subject pronouns unless they are needed for emphasis or contrast.

• Często o niej myślę. = I often think about it.
• Ja często o niej myślę. = I often think about it (with extra emphasis on I)

Myślę is the 1st person singular present tense of the verb myśleć = to think.

So:

• myśleć = to think
• myślę = I think

A few present-tense forms are:

• myślę = I think
• myślisz = you think
• myśli = he/she/it thinks
• myślimy = we think
• myślicie = you (plural) think
• myślą = they think

In this sentence, często o niej myślę means I often think about it/her.

Yes. Polish word order is much more flexible than English word order, because case endings and verb forms carry a lot of grammatical information.

Your sentence:

• Ta książka jest ciekawa, często o niej myślę.

Other natural possibilities include:

• Ta książka jest ciekawa, często myślę o niej.
• O niej często myślę.
• Często myślę o niej.

These versions are all grammatical, but the emphasis changes slightly.

For example:

• często o niej myślę sounds neutral
• o niej często myślę puts more focus on about it
• często myślę o niej is also very natural and common

Yes, in normal standard Polish, jest is the usual form here.

• Ta książka jest ciekawa. = This book is interesting.

Polish does sometimes omit forms of to be in very informal, poetic, or headline-like language, but for regular everyday sentences, you should learn and use:

• X jest ciekawy / ciekawa / ciekawe

So for a learner, jest should definitely be included here.

Because the sentence contains two closely connected clauses:

• Ta książka jest ciekawa
• często o niej myślę

The comma shows a pause and links two related thoughts. In English, you would often separate these ideas with a conjunction or make them two sentences. In Polish, this kind of close connection is common.

You could also write them as two separate sentences:

• Ta książka jest ciekawa. Często o niej myślę.

That would also be natural.

Because często is an adverb meaning often, and in Polish adverbs can move around more freely than in English.

Here it modifies the whole idea of thinking about it:

• często o niej myślę = I often think about it

Other possible placements are:

• o niej często myślę
• myślę o niej często

All are possible, but they may sound slightly different in emphasis. The version in your sentence is very natural and neutral.

Not with the same meaning.

• myśleć o czymś / o kimś = to think about something / someone
• myśleć nad czymś = to think over something, consider, reflect on a problem

So:

• Często o niej myślę. = I often think about it.
• Myślę nad tą książką. would sound more like I am reflecting on this book / thinking it over, perhaps analyzing it

So in your sentence, o niej is the correct and most natural choice.