Learn Hardcore Croatian: Ispod starog stabla u parku često sanjam isti san o mirnom moru. - Under the old tree in the park I often dream the same dream about a calm sea.

The ending -am tells you it is 1st person singular, so the subject must be I. The pronoun ja is understood and normally left out unless you want to stress I.

Why is it ispod starog stabla and not ispod staro stablo?

Because of the preposition ispod.

ispod (under, beneath) always takes the genitive case.
The noun stablo (tree) in nominative is stablo (what?):
- nominative: staro stablo – an old tree
In the genitive singular, stablo becomes stabla (of an old tree):
- genitive: starog stabla

So after ispod you must use the genitive: ispod starog stabla (under the old tree), not nominative staro stablo.

Why does the adjective change to starog instead of staro in starog stabla?

Adjectives in Croatian agree with the noun in gender, number, and case.

Noun stablo is:
- neuter
- singular
- here: genitive because of ispod
The basic adjective form is:
- staro stablo – nominative neuter singular

In genitive singular for neuter (and masculine), the adjective ends in -og:

nominative: staro stablo (an old tree – subject)
genitive: starog stabla (of an old tree / under an old tree)

So starog is just the genitive form matching stabla.

Why is it u parku and not u park for in the park?

The preposition u can take two different cases:

u + accusative – motion into something (where to?)
- Idem u park. – I am going to the park.
u + locative – location inside something (where?)
- Sam u parku. – I am in the park.

In your sentence the park is a location, not a destination, so u requires the locative:

Nominative: park
Locative: parku

Therefore we say u parku here.

What case is isti san, and why is it not istog sna?

The verb sanjati takes a direct object in the accusative case.

Noun san (dream) is masculine:
- nominative: san
- genitive: sna
- accusative: san (same as nominative for inanimate masculine)

In your sentence, san is the object of sanjam, so it is in the accusative: san.

The adjective isti (same) must match:

nominative m. sg.: isti san
accusative m. sg. inanimate: also isti san

istog sna would be genitive (of the same dream), which is not needed here because the verb requires accusative, not genitive. So sanjam isti san is correct.

Where does the -am in sanjam come from, and what does it tell me?

The verb sanjati (to dream) in the present tense has these endings:

ja sanjam
ti sanjaš
on/ona sanja
mi sanjamo
vi sanjate
oni/one sanjaju

The ending -am marks:

present tense
1st person singular (I)

This is why the pronoun ja can be dropped: sanjam already means I dream.

Why is često placed before sanjam? Could I put it somewhere else?

često is an adverb meaning often, and the most neutral place for adverbs of frequency is before the verb:

Često sanjam isti san… – I often dream the same dream…

Other positions are also possible, with slight changes in emphasis:

Ispod starog stabla u parku često sanjam isti san o mirnom moru.
(As in your sentence – neutral, smooth.)
Često ispod starog stabla u parku sanjam isti san o mirnom moru.
(Slight extra emphasis on how often, right from the start.)
Sanjam često isti san…
(Unusual but possible; now često is more strongly connected to sanjam as a comment: I dream, and I do it often.)

So yes, često can move, but before the main verb is the most typical position.

Why does more become moru in o mirnom moru?

The noun more (sea) is neuter. Its singular forms:

nominative: more
genitive: mora
dative/locative: moru
accusative: more
instrumental: morem

The preposition o (about) takes the locative case:

o čemu? – about what?
o moru – about the sea

So more changes to moru because it is in the locative after o.

Why is it mirnom moru and not mirno more?

Again, because of case agreement.

The adjective mirno (calm) is:
- nominative neuter singular: mirno more – a calm sea
After o, the noun more goes to locative moru
The adjective must match that case, gender and number:
- locative neuter singular: mirnom moru

So:

nominative: mirno more (subject, basic form)
locative: o mirnom moru (about the calm sea)

What does o mean in o mirnom moru, and which case does it take?

Here o means about (on the topic of).

It always takes the locative case:
- pričam o filmu – I am talking about the film
- razmišljam o tebi – I think about you
- sanjam san o mirnom moru – I dream a dream about the calm sea

So o mirnom moru is about a calm sea, with mirnom moru in the locative.

Can I change the word order of the whole sentence? For example: Često sanjam isti san o mirnom moru ispod starog stabla u parku.

Yes. Croatian word order is quite flexible, especially for elements like adverbs and prepositional phrases.

All of these are grammatically correct:

Ispod starog stabla u parku često sanjam isti san o mirnom moru.
(Your original – starts with the place, then frequency and action.)
Često sanjam isti san o mirnom moru ispod starog stabla u parku.
(Starts with how often, then what you dream, then where.)
Sanjam isti san o mirnom moru ispod starog stabla u parku, i to često.
(More emphasis on the content of the dream first.)

The main rules you must keep are:

keep each preposition with its noun phrase (ispod starog stabla, u parku, o mirnom moru),
keep the verb and its object together (sanjam isti san),
use the correct cases regardless of word order.

Changing the order mostly affects which part of the sentence feels most emphasized or topical.