Learn Hardcore Croatian: Želim ti zahvaliti na pomoći.

Croatian infinitives typically end in -ti (zahvaliti, pisati, doći).
After verbs of desire, intention, or ability, Croatian often uses the infinitive, just like English:
- Želim spavati. = I want to sleep.
- Želim ti zahvaliti. = I want to thank you.

There is also an aspect difference in Croatian:

zahvaljivati – imperfective (an ongoing or repeated action)
zahvaliti – perfective (a single, completed act)

Here you say Želim ti zahvaliti because you are talking about doing one act of thanking, not a repeated habit of thanking.

Why is it zahvaliti na pomoći, not zahvaliti za pomoć?

With zahvaliti, standard Croatian uses the pattern:

zahvaliti (se) nekome na nečemu
→ thank someone for something

So the correct preposition is na + locative:

zahvaliti na pomoći = thank for the help
zahvaliti na podršci = thank for the support
zahvaliti na dolasku = thank for coming

Using za here (zahvaliti za pomoć) is non‑standard or at least feels wrong to most native speakers in formal Croatian.

What case is pomoći in, and why doesn’t it look like pomoć?

Here pomoći is the locative singular of the noun pomoć (help).

Declension of pomoć (feminine noun, like noć – night):

Nominative: pomoć (help)
Genitive: pomoći
Dative: pomoći
Accusative: pomoć
Locative: pomoći
Instrumental: pomoći

With na in this meaning (on / for something), you use locative, so:

na pomoći = on/for the help (locative)

Complication: pomoći is also the infinitive of the verb pomoći (to help). So the form pomoći can be:

a verb infinitive: Želim ti pomoći. = I want to help you.
a noun in oblique cases: Hvala ti na pomoći. = Thank you for the help.

In Želim ti zahvaliti na pomoći, it’s the noun (help), locative case, after na.

Why is the pronoun ti in the middle: Želim ti zahvaliti, and not Želim zahvaliti ti?

Because ti here is a clitic pronoun, and Croatian clitic pronouns almost always stand in second position in the clause.

Very simplified rule:

The clitic (like ti, mi, mu, se, ga, je) normally comes right after the first stressed word or phrase in the sentence.

In your sentence:

First stressed word: Želim
Then comes the clitic: ti
Then the rest: zahvaliti na pomoći

So:

Želim ti zahvaliti na pomoći. ✅ natural
Želim zahvaliti ti na pomoći. ❌ sounds wrong / foreign
Ja ti želim zahvaliti na pomoći. ✅ also correct (first stressed word is Ja, then clitic ti, then želim…)

This “second position” rule is one of the big word‑order differences between English and Croatian.

Could I also say Želim ti se zahvaliti na pomoći? What’s the difference between zahvaliti and zahvaliti se?

Yes, you can say:

Želim ti zahvaliti na pomoći.
Želim ti se zahvaliti na pomoći.

Both are correct and very common, and in most everyday situations they mean the same thing: I want to thank you for your help.

Details:

zahvaliti nekome na nečemu
zahvaliti se nekome na nečemu

The se is a reflexive pronoun. In this verb it does not change the basic meaning much; many speakers use both forms almost interchangeably when they mean “to thank”.

A side note: in some contexts, zahvaliti se na nečemu can also mean to politely refuse/decline something (like “thank you, but no”):

Zahvalio sam se na ponudi.
= I politely declined the offer.

In your sentence of gratitude, though, both versions are fine and understood simply as thanking.

Can I drop ti and just say Želim zahvaliti na pomoći?

Grammatically, yes, you can omit ti:

Želim zahvaliti na pomoći.
= I want to give thanks for the help.

But the nuance changes:

With ti: you clearly say who you are thanking: you.
- Želim ti zahvaliti na pomoći. = I want to thank you for your help.
Without ti: it’s more general and less personal. It sounds like a formal statement, for example in a speech:
- Želim zahvaliti na pomoći svim kolegama.
  = I want to thank all my colleagues for the help.

In a direct conversation with one person, native speakers would almost always keep ti (or formal Vam).

How do I make this sentence polite/formal when speaking to someone I address as Vi?

You change ti (informal singular you) to vam/Vam (dative of vi = you plural/formal):

Želim Vam zahvaliti na pomoći.

Notes:

vam is the dative clitic pronoun for vi.
In formal letters and emails, Croatians often write it with a capital V (Vam) as a sign of respect.
This works exactly like ti:
- Želim ti zahvaliti na pomoći. (informal you)
- Želim vam zahvaliti na pomoći. (spoken formal/plural)
- Želim Vam zahvaliti na pomoći. (formal, in writing)

Could I say Želim da ti zahvalim na pomoći instead of Želim ti zahvaliti na pomoći?

You can hear:

Želim da ti zahvalim na pomoći.

It is grammatically understandable and used in speech, but for standard Croatian, the version with the infinitive is usually preferred, especially in writing:

Želim ti zahvaliti na pomoći. ✅ more standard/neutral

Differences:

Želim ti zahvaliti… → verb of desire (želim) + infinitive (zahvaliti)
Želim da ti zahvalim… → verb (želim) + da‑clause (da ti zahvalim)

In everyday conversation, many people do use da‑clauses, but if you want to stay close to the textbook / standard Croatian model, stick with:

Želim ti zahvaliti na pomoći.

Is there a simpler or more common way to express the same idea?

Yes, very common alternatives include:

Hvala ti na pomoći.
= Thank you for your help.
(Probably the most frequent, straightforward way.)
Puno ti hvala na pomoći.
= Thank you very much for your help.
More formal:
- Hvala Vam na pomoći.
- Puno Vam hvala na pomoći.
- Želio bih Vam zahvaliti na pomoći.
  (I would like to thank you for your help – extra polite.)

Your original sentence Želim ti zahvaliti na pomoći is correct and sounds somewhat intentional / slightly formal, like you are explicitly announcing that you want to express thanks.