Learn Hardcore Croatian: Sljedeće godine planiram ostati u gradu, a ne ići na more. - Next year I am planning to stay in the city and not go to the sea.

Nominative: sljedeća godina = the next year (as the grammatical subject).
Genitive: sljedeće godine = literally of next year, but idiomatically it means next year on its own.

In expressions of time, Croatian often uses the genitive:

prošle godine – last year
sljedeće godine / iduće godine – next year
ovog ljeta – this summer

So here sljedeće agrees with godine in gender (f), number (sg), and case (genitive), and the whole phrase functions adverbially: when? – sljedeće godine (next year).

Could I also say Planiram sljedeće godine ostati u gradu or Planiram ostati u gradu sljedeće godine?

Yes. All of these are grammatically correct:

Sljedeće godine planiram ostati u gradu, a ne ići na more.
Planiram sljedeće godine ostati u gradu, a ne ići na more.
Planiram ostati u gradu sljedeće godine, a ne ići na more.

The difference is mainly emphasis:

At the beginning: Sljedeće godine is highlighted (the time is important).
In the middle / end: the focus feels more on what you plan (staying in the city instead of going to the seaside), with the time as extra information.

Croatian word order is fairly flexible; you mostly adjust it for what you want to stress.

Why is it planiram ostati and not something like planiram da ostanem?

In standard Croatian, after planirati (to plan) you normally use the infinitive:

planiram ostati – I plan to stay
planiramo putovati – we plan to travel

Using da + present tense (planiram da ostanem) is not the usual pattern with planirati in standard language. You will hear da-clauses more with verbs like želim da ostanem, nadam se da ću ostati, but with planirati, the infinitive is the natural choice.

So: planiram ostati = perfect, idiomatic Croatian.

Why is the verb ostati (perfective) used instead of ostajati (imperfective)?

Croatian uses perfective verbs (like ostati) when talking about:

a single, completed action or state in the future
a concrete plan or decision

Here, you’re talking about one whole period (next year) as a single “stay” event, so ostati fits:

planiram ostati u gradu – I plan to (stay / remain) in the city (for that period).

Ostajati is the imperfective form and would suggest a repeated or ongoing process (to be staying repeatedly / habitually). In this sentence, that nuance is not needed; it’s one planned, bounded situation, so perfective ostati is preferred.

Why is it u gradu and not u grad?

The preposition u takes:

Accusative for movement into something:
- ići u grad – to go to the city
Locative for location inside something:
- biti u gradu – to be in the city

After ostati (to stay/remain), we are talking about location, not movement. So we use the locative:

ostati u gradu – to stay in the city (locative)
ići u grad – to go to the city (accusative)

U gradu here is locative singular of grad.

What does na more literally mean, and why not u more or na moru?

Literally:

more = the sea (neuter noun)
na more (accusative) = onto / to the sea

But in everyday Croatian:

ići na more almost always means “go to the seaside / go to the coast (for a holiday)”.

Compare:

ići na more – go to the seaside (vacation context)
biti na moru – be at the seaside
u more – into the water of the sea (e.g. skakati u more – to jump into the sea)

So na more here is a set phrase: it doesn’t just mean physically moving toward water; it means going on a seaside holiday.

Why is the second verb ići also in the infinitive: a ne ići na more?

Because it depends on the same verb planiram:

planiram [ostati u gradu]
(planiram) [ne ići na more]

In Croatian, when you have two actions connected (stay in the city vs. go to the seaside) after a verb like planirati, you keep them in parallel infinitive form, even if the main verb is only written once:

planiram jesti povrće, a ne piti gazirana pića
planiram učiti, a ne gledati televiziju

Using da ne idem would be a different construction, and it would break that parallelism. A ne ići is the normal, concise way to say rather than go / and not to go here.

What does the conjunction a add here? How is a ne ići na more different from i ne ići na more or nego ići na more?

A is a coordinating conjunction often used for contrast (milder than ali):

ostati u gradu, a ne ići na more
≈ stay in the city, and (instead) not go to the seaside.

Nuances:

a ne – sets up an alternative/contrast: this, rather than that.
i ne ići na more – would sound more like and also not go to the seaside (adding another negative action, weaker contrast).
nego ići na more – usually follows a negative sentence and introduces the corrected option:
- Neću ostati u gradu, nego ću ići na more. – I won’t stay in the city, but rather I’ll go to the seaside.

So in this sentence, a ne is perfect: it contrasts staying in the city with not going to the seaside.

Why is there a comma before a?

In Croatian, when a connects two independent clauses or two longer coordinated parts, you usually put a comma before it:

… planiram ostati u gradu, a ne ići na more.

Think of it as separating two coordinated ideas:

planiram ostati u gradu
(planiram) ne ići na more

So the comma is normal and expected there.

Why is planiram in the present tense when the meaning is future (next year)?

Croatian often uses the present tense to talk about planned or scheduled future actions, especially with verbs of intention like:

planirati – to plan
namjeravati – to intend
željeti – to want

Planiram ostati… literally is I am planning to stay…, but in context with sljedeće godine it clearly refers to a future plan.

You could also say:

Sljedeće godine ću ostati u gradu, a ne ići na more. – Next year I will stay… (neutral future)
Sljedeće godine planirat ću ostati u gradu… – Next year I will plan to stay… (different meaning: the planning will happen next year).

In the original, the planning is now, the staying is in the future. That’s why present planiram works naturally.

Could I shorten it to Sljedeće godine planiram ostati u gradu, a ne na more?

Yes, in everyday speech you can omit the second verb and say:

Sljedeće godine planiram ostati u gradu, a ne na more.

Here, ići is understood from context:

a ne (ići) na more

This kind of ellipsis is common and sounds natural in casual conversation. The full version (a ne ići na more) is a bit clearer and more explicit, so it’s often preferred in writing or more careful speech.

Is there any difference between sljedeće godine and iduće godine?

They are essentially synonyms:

sljedeće godine – next year
iduće godine – next year

Both are very common and standard. Small nuances:

Some speakers feel idući is slightly more colloquial in some regions, but both forms are widely accepted.
In many contexts they are completely interchangeable:
- Sljedeće godine planiram…
- Iduće godine planiram…

For learning purposes, you can treat them as meaning the same.