Learn Hardcore Croatian: Ne želim usporediti ovaj film i onaj film. - I do not want to compare this movie and that movie.

That sounds wrong or at best extremely marked. The negation targets your wanting (you don’t want), not the action of comparing itself in a direct grammatical way, so ne must go with želim.

Other examples with the same pattern:

Ne mogu razumjeti. – I cannot understand.
Ne želim ići. – I don’t want to go.
Ne smijem govoriti. – I must not speak.

What is the difference between ne želim and neću?

Both can be translated as I won't in some contexts, but they are not the same:

ne želim = I don’t want (to)
- Focuses on your desire / will.
- Neutral, often slightly softer.
neću (from htjeti) = literally I will not
- Can be stronger, more categorical refusal.
- Also used as the future tense auxiliary (ću) in positive sentences.

Examples:

Ne želim usporediti ovaj film i onaj film.
I don’t want to compare this film and that film. (statement of preference)
Neću uspoređivati ova dva filma.
I won’t be comparing these two films. (more like a decision / firm refusal)

In polite or soft speech, ne želim often sounds less harsh than neću.

Why is usporediti in the infinitive form after želim?

Verbs of wanting, needing, being able, etc., in Croatian are typically followed by a bare infinitive:

želim usporediti – I want to compare
moram učiti – I have to study
mogu pomoći – I can help

So the pattern is:

[finite verb] + [infinitive]

In your sentence:

želim is finite (1st person singular).
usporediti is the infinitive.

You can also express the idea with a da + finite verb construction, but it sounds a bit heavier:

Ne želim da uspoređujem ovaj film i onaj film.
Grammatically OK, but less natural here; native speakers would usually prefer the infinitive:
Ne želim usporediti ovaj film i onaj film.

What is the difference between usporediti and uspoređivati?

This is a verb aspect difference: perfective vs imperfective.

usporediti – perfective
- Focus on a single, complete act of comparing.
- Used when you talk about doing it once / as a whole action.
- Often used after želim, moram, hoću when you mean “do it (once)”.
uspoređivati – imperfective
- Focus on the process, repetition, or ongoing nature of comparing.
- Means to be comparing, to keep comparing, to compare repeatedly / habitually.

Compare:

Ne želim usporediti ovaj film i onaj film.
I don’t want to (even once) compare this film and that film.
Ne želim uspoređivati ove filmove.
I don’t want to (keep) comparing these films / I don’t want to be in the habit of comparing these films.

Both are correct; you choose based on whether you want to stress a single act or an ongoing activity.

Which case are ovaj film and onaj film in, and why?

They are in the accusative singular, because they are direct objects of the verb usporediti (to compare).

For the noun film (masculine, inanimate):

Nominative singular: film
Accusative singular: film (same form as nominative, because it’s inanimate)

For the demonstratives:

ovaj (this) – nominative masculine singular: ovaj
Accusative masculine singular (inanimate) is also ovaj.
onaj (that over there / that one) – nominative masculine singular: onaj
Accusative masculine singular (inanimate) is also onaj.

So:

usporediti koga/što? – ovaj film, onaj film (accusative objects)

What exactly is the difference between ovaj and onaj here?

Croatian has three basic demonstratives, more fine‑grained than English:

ovaj – this (near the speaker)
taj – that (near the listener or just mentioned)
onaj – that (over there) / that one (farther away, more remote)

In your sentence:

ovaj film – the movie that is close to the speaker (physically, or in focus now).
onaj film – the movie that is farther away, or more psychologically distant, or previously mentioned in contrast.

So:

Ne želim usporediti ovaj film i onaj film.
Can be understood as: I don’t want to compare this movie (here) with that other one (over there / further away / the other one we mentioned).

In many contexts, English just uses this and that, but Croatian keeps a clearer three‑way distinction.

Do I have to repeat the word film, or can I say ovaj i onaj film?

You have several correct options:

Ne želim usporediti ovaj film i onaj film.
(Full repetition – very clear, slightly more formal/neutral.)
Ne želim usporediti ovaj i onaj film.
(Omit film after ovaj; film is understood from context. Very natural.)
Ne želim usporediti ovaj film i onaj.
(Omit film after onaj; again film is understood. Also natural.)

All three are grammatical.
Stylistically, Croatian often avoids unnecessary repetition, so (2) and (3) are extremely common in spoken language.

If the context isn’t clear (for example, many kinds of things are being discussed), repeating film can be safer.

Can I change the word order, for example: Ovaj film i onaj film ne želim usporediti?

Yes. Croatian has relatively flexible word order, and you can move elements for emphasis or focus. All of these are grammatical, but have slightly different information focus:

Ne želim usporediti ovaj film i onaj film.
Neutral: focus on the not wanting.
Ovaj film i onaj film ne želim usporediti.
Fronts the objects; emphasis on which things you don’t want to compare. Roughly:
It’s these two films that I don’t want to compare.
Ne želim ovaj film i onaj film usporediti.
Possible, but feels a bit marked / less natural; verb + infinitive usually stay closer together.

Default, most neutral order is the original: Ne želim usporediti ovaj film i onaj film.

Could I say Ne želim usporediti ova dva filma instead? What’s the difference?

Yes, that’s a very natural alternative:

Ne želim usporediti ovaj film i onaj film.
Ne želim usporediti ova dva filma. – I don’t want to compare these two films.

Differences:

ovaj film i onaj film explicitly contrasts this vs that film.
ova dva filma just presents them as two specific films, grouped together, without highlighting the contrast as strongly.

Also note the grammar:

ova dva filma
- ova – nominative/accusative neuter plural of ovaj/ovaj/ovo (agrees with dva here)
- dva – “two”
- filma – genitive singular form used after dva (two films)

When should I use i and when should I use ni in this kind of sentence?

i = and (positive coordination)
ni = nor / and not (negative coordination, used with ne)

In your sentence, you are negating the verb:

Ne želim usporediti ovaj film i onaj film.
The wanting to compare is negated; you’re just listing the two objects with i.

If you want to emphasize that you don’t want to compare either one, you can use ni ... ni together with ne:

Ne želim usporediti ni ovaj film ni onaj film.
Literally: I do not want to compare neither this film nor that film.
(In English: I don’t want to compare either this film or that film.)

Important: in standard Croatian, ne is kept with ni ... ni (double negation is normal):

Ne želim ni ovo ni ono. – I want neither this nor that.

Is there a reflexive form like usporediti se and does it mean something different?

Yes, usporediti se is the reflexive form and it usually means “to compare oneself” (often with someone/something).

usporediti – to compare (A with B)
- Ne želim usporediti ovaj film i onaj film. – I don’t want to compare this film and that film.
usporediti se (s/sa + instrumental) – to compare oneself (with)
- Ne želim se uspoređivati s drugim ljudima. – I don’t want to compare myself with other people.

So usporediti se would not be used in your movie‑comparison sentence; you need the non‑reflexive usporediti there.

Is this sentence polite/neutral, or does it sound rude?

Ne želim usporediti ovaj film i onaj film. is neutral in tone. It simply states your preference: you don’t want to compare them.

To make it softer or more polite, you can:

Use a conditional form:
- Ne bih želio uspoređivati ovaj film i onaj film.
  (male speaker; more formal/polite)
- Ne bih htjela uspoređivati ovaj film i onaj film.
  (female speaker)
Or add a softening phrase, e.g.:
- Radije ne bih uspoređivao/uspoređivala ova dva filma. – I’d rather not compare these two films.

But as written, it does not sound rude; it’s a straightforward, neutral sentence.