Learn Hardcore Croatian: Večeras ću platiti stanarinu gotovinom.

the appropriate form of htjeti (to want) used as a clitic (ću, ćeš, će, ćemo, ćete, će)
- the infinitive of the main verb (here: platiti = to pay (once, to complete the action))

So:

Ja ću platiti = I will pay

In your sentence, ću marks future tense, and platiti is the infinitive of the verb platiti / plaćati (to pay). Together they mean I will pay (with a completed, one‑time act of paying).

Can I change the word order of ću and platiti? For example, is platit ću also correct?

Yes, Croatian allows both orders, but with some rules and nuances.

Neutral, most common written form (especially in standard style):
- Večeras ću platiti stanarinu gotovinom.
Also correct and very common in speech:
- Večeras ću platit stanarinu gotovinom.
- Večeras platit ću stanarinu gotovinom. (possible, but sounds more marked/emphatic)

Notes:

Ću is a clitic and normally appears in second position in the clause or right after the first stressed element (here večeras).
Platiti can lose the final -i in speech (platit), especially before ću, but in careful writing platiti is preferred.
*platiti ću (infinitive + ću written as two separate words in that order) is considered wrong in standard Croatian. You either use ću platiti or the shortened infinitive
- ću (platit ću).

Why is it stanarinu instead of stanarina?

Stanarina is a feminine noun meaning rent (for accommodation).

In the sentence, stanarinu is the direct object of the verb platiti (what will I pay?), so it must be in the accusative singular case.

Feminine nouns ending in -a typically decline like this (singular):

Nominative (dictionary form): stanarina – rent (as the subject)
Accusative (direct object): stanarinu – rent (as the thing being paid)

So:

Stanarina je skupa. – The rent is expensive. (subject, nominative)
Platiti ću stanarinu. – I will pay the rent. (object, accusative)

That’s why you see stanarinu here.

What exactly does gotovinom mean, and why is it in that form?

Gotovina means cash.

Gotovinom is the instrumental singular form of gotovina and here it means with cash / in cash.

Instrumental case is often used to express the means or instrument of an action:

Pisati olovkom. – to write with a pencil
Platiti karticom. – to pay by card
Platiti gotovinom. – to pay in cash

You can also say:

platiti u gotovini – literally to pay in cash (using a preposition + locative)

Both gotovinom and u gotovini are common; gotovinom is a bit shorter and very natural in speech.

Is it necessary to say ja (I), or is Večeras ću platiti... enough?

Ja is not necessary here. Croatian is a pro‑drop language: the verb form already shows the subject.

Večeras ću platiti stanarinu gotovinom. – completely natural
Ja ću večeras platiti stanarinu gotovinom. – also correct, but ja adds emphasis: I (as opposed to someone else) will pay.

So you usually omit ja unless you want to stress who is doing the action.

Why is the verb platiti and not plaćati? What’s the difference?

Croatian has aspect: most verbs come in pairs:

plaćati – imperfective (ongoing, repeated, process)
platiti – perfective (single, completed action/result)

In your sentence you’re talking about one specific payment that will be completed this evening, so the perfective platiti is the natural choice.

Compare:

Večeras ću platiti stanarinu. – I’ll (get it done and) pay the rent this evening.
Večeras ću plaćati stanarinu. – would sound like This evening I’ll be in the process of paying rent / making rent payments (very odd in normal context).

For planned, single, future actions with a clear endpoint, Croatian usually uses the perfective form (platiti).

Can I move the words around? For example, could I say Stanarinu ću večeras platiti gotovinom?

Yes, Croatian word order is quite flexible, and all of these are grammatical, with slight changes in emphasis:

Večeras ću platiti stanarinu gotovinom. – neutral: focus on when.
Stanarinu ću večeras platiti gotovinom. – puts focus on stanarinu (the rent).
Gotovinom ću večeras platiti stanarinu. – emphasizes gotovinom (the fact it’s in cash).
Večeras stanarinu ću platiti gotovinom. – more marked/emphatic order; used for special emphasis or in spoken, expressive style.

The basic, most neutral version is the one you started with. Changing the order doesn’t usually change meaning, but it can change what you’re highlighting in the sentence.

What is the exact difference between večeras, večer, and noćas?

Večeras – this evening (adverb). Refers to tonight in the evening hours, typically before late night.
- Večeras ću biti doma. – I’ll be home this evening.
Večer – evening as a noun.
- Ova večer je lijepa. – This evening is nice.
- Uvečer – in the evening (adverbial form using the noun).
Noćas – tonight (during the night), usually later than what you’d call večeras; more like the night‑time hours.
- Noćas nisam spavao. – I didn’t sleep last night/tonight.

In your sentence, večeras is right, because paying rent is something you’d expect to do in the evening, not in the middle of the night.

How do I make this sentence negative? How do I say I will not pay the rent in cash this evening?

To negate the Croatian future, you:

combine ne
- the auxiliary (ću) into neću, nećeš, etc.
keep the main verb in the infinitive.

So:

Večeras neću platiti stanarinu gotovinom. – I will not pay the rent in cash this evening.

Note:

ne + ću → neću (written as one word)
word order is similar; the negation usually attaches to the auxiliary, not to the main verb.

How do I pronounce ću and večeras?

Ću
- Ć is a palatal sound, somewhat like the ch in cheap, but softer and produced further forward in the mouth.
- Ću sounds roughly like “choo”, but with a softer ch.
Večeras
- Syllables: ve‑če‑ras.
- č is like ch in chocolate.
- Approximate pronunciation: VEH‑che‑ras (stress usually on the first syllable in standard pronunciation: VE‑čeras).

So the whole sentence would roughly sound like: VEH‑che‑ras chu PLAH‑tee‑tee stah‑NAH‑ree‑noo goh‑TOH‑vee‑nom (with Croatian vowels always short and clear).