Learn Hardcore Polish: Nie wiem, czy ona lubi ten film. - I don’t know if she likes this movie.

So Nie wiem literally is Don’t know, but it is always understood as I don’t know.
You can say Ja nie wiem for emphasis (like I don’t know), but the neutral version is simply Nie wiem.

What exactly does czy mean here?

czy in this sentence is a conjunction meaning roughly whether / if in I don’t know whether/if she likes this film.

Key points:

It introduces an indirect yes/no question:
- Nie wiem, czy ona lubi ten film. = I don’t know whether she likes this film.
On its own, at the beginning of a sentence, czy is often a question word:
- Czy ona lubi ten film? = Does she like this film?

So here czy links the verb wiem with the clause ona lubi ten film and turns it into a whether/if-type clause.

Why is there a comma before czy?

In Polish, you normally put a comma before conjunctions that introduce a subordinate clause, including czy in this kind of structure.

Nie wiem, czy ona lubi ten film.

Here:

Nie wiem = main clause
czy ona lubi ten film = subordinate clause (what I don’t know)

Subordinate clauses in Polish are almost always preceded by a comma, so the comma is mandatory here.

Why is there no question mark? This feels like a question in English.

The whole sentence is actually a statement, not a direct question.

Direct question (with a question mark):
- Czy ona lubi ten film? = Does she like this film?
Indirect question (embedded in a statement):
- Nie wiem, czy ona lubi ten film. = I don’t know whether she likes this film.

In English, we also write I don’t know whether she likes this film. with a period, not a question mark. Polish follows the same logic: only direct questions take a question mark.

Can I change the word order to Nie wiem, ona czy lubi ten film or Nie wiem, czy lubi ona ten film?

Nie wiem, ona czy lubi ten film – this is wrong. czy must come immediately before the clause it introduces.
Nie wiem, czy lubi ona ten film – this is grammatically possible, but sounds marked or slightly poetic; it puts extra emphasis on ona (like stressing she in English).

The neutral, everyday order is:

Nie wiem, czy ona lubi ten film.

You may also drop the pronoun:

Nie wiem, czy lubi ten film. – also perfectly natural.

What is the role of ona here? Do I have to use it?

ona means she and is the subject of lubi.

ona lubi = she likes

In Polish you can often omit subject pronouns because the verb ending shows the person:

Nie wiem, czy lubi ten film. – understood as I don’t know whether she/he likes this film.

So:

Use ona if you need to specify it’s a woman, or for emphasis.
Omit it if context already makes clear who you’re talking about.

Why is the verb lubi, not something like lubię or lubią?

lubi is the 3rd person singular present tense form of lubić (to like).

Present tense of lubić:

ja lubię – I like
ty lubisz – you (sg.) like
on/ona/ono lubi – he/she/it likes
my lubimy – we like
wy lubicie – you (pl.) like
oni/one lubią – they like

Because the subject is ona (she), you must use lubi:

ona lubi ten film = she likes this film

What is the difference between lubić and polubić?

Both are related to liking, but aspect and meaning differ:

lubić – imperfective, describes a general, ongoing liking:
- Ona lubi ten film. – She likes this film. (in general)
polubić – perfective, means to come to like / to start liking:
- Ona polubiła ten film. – She came to like this film / grew to like it.

In your sentence, we’re talking about a general preference, so lubi (from lubić) is correct:

Nie wiem, czy ona lubi ten film. – I don’t know whether she (generally) likes this film.

What tense is lubi? Could I say lubiła instead?

lubi is present tense:

ona lubi = she likes / she is liking (English usually just likes)

lubiła is past tense, 3rd person singular feminine:

Ona lubiła ten film. – She used to like / liked this film.

So if you want to talk about her liking the film now / in general, you use lubi:

Nie wiem, czy ona lubi ten film. – I don’t know whether she (now/usually) likes this film.
Nie wiedziałem, czy ona lubiła ten film. – I didn’t know whether she liked that film. (past context)

What case is ten film in, and why?

ten film is in the accusative case, because it is the direct object of the verb lubi.

Verb: lubi – likes
Direct object: ten film – this film

For a masculine inanimate noun like film, the accusative singular form is the same as the nominative:

Nominative: ten film (this film – as subject)
Accusative: ten film (this film – as object)

So the form doesn’t change here, but grammatically it functions as accusative.

Why ten film and not something like tego filmu?

Both are correct forms, but they are different cases:

ten film – nominative / accusative (used here as accusative, direct object)
tego filmu – genitive

You would use tego filmu in contexts that require the genitive, for example:

after some prepositions:
- bez tego filmu – without this film
after negated verbs with some verbs:
- Nie oglądam tego filmu. – I am not watching this film.

With lubić, the normal object is accusative, so we say:

Ona lubi ten film. – She likes this film.
Nie wiem, czy ona lubi ten film. – I don’t know whether she likes this film.

Why is nie before wiem? Can I say Wiem nie?

Negation with nie almost always goes directly before the verb in Polish:

wiem → nie wiem
rozumiem → nie rozumiem
lubi → nie lubi

Wiem nie is incorrect in standard Polish. The natural order is:

Nie wiem, czy ona lubi ten film.

If you want to negate lubi, you also put nie before it:

Nie wiem, czy ona nie lubi tego filmu. – I don’t know whether she doesn’t like this film.

Is czy always necessary in this kind of sentence? Could I say Nie wiem, ona lubi ten film?

If you keep the current structure, czy is necessary. Without it:

Nie wiem, ona lubi ten film.

…sounds like two separate sentences stuck together: I don’t know. She likes this film.

To make it one sentence meaning I don’t know whether she likes this film, you need czy (or another conjunction) to introduce the subordinate clause:

Nie wiem, czy ona lubi ten film.

You can change the structure entirely, though:

Nie wiem, czy ona lubi ten film. – I don’t know whether she likes this film.
Nie wiem, lubi ona ten film czy nie. – I don’t know, does she like this film or not. (more colloquial, spoken style)

What is the difference between czy and jeśli / jeżeli (if)?

They translate into English as if in different senses:

czy = whether / if in indirect questions and yes/no questions:
- Nie wiem, czy ona lubi ten film. – I don’t know whether she likes this film.
jeśli / jeżeli = if in conditions:
- Jeśli ona lubi ten film, to go obejrzymy. – If she likes this film, we’ll watch it.

You cannot replace czy with jeśli in your sentence:

Nie wiem, jeśli ona lubi ten film. – incorrect / very unnatural.

Is film just the English word film borrowed into Polish? Does it decline like a normal Polish noun?

Yes, film is a loanword, but it behaves like a regular Polish masculine inanimate noun and fully declines:

Nominative: film – Ten film jest ciekawy.
Genitive: filmu – Nie znam tego filmu.
Dative: filmowi – Przyglądam się filmowi.
Accusative: film – Oglądam ten film.
Instrumental: filmem – Jestem zachwycony tym filmem.
Locative: filmie – Myślę o tym filmie.

So in your sentence, ten film is just the accusative form of a normal Polish noun.