Learn Hardcore Croatian: Kažu da naša budućnost ovisi o tome kakve odluke donosimo danas, u sadašnjosti. - They say that our future depends on what decisions we make today, in the present.

Kažu is the 3rd person plural present of kazati (to say), literally they say.
In Croatian, this 3rd person plural form is often used impersonally, meaning people say / it is said.
You could say Oni kažu… or Ljudi kažu…, but in general statements like this, the subject is usually dropped and just Kažu… is very natural.

Is da in Kažu da naša budućnost… the same as English that? Can it be omitted?

Yes. Here da introduces a subordinate clause and works like English that in They say that our future depends….
Unlike English, in Croatian you cannot omit it in this structure:

Correct: Kažu da naša budućnost ovisi…
Incorrect: ✗ Kažu naša budućnost ovisi…
So you need da to link Kažu with the whole idea that follows.

Why is it naša budućnost, not naš budućnost?

Croatian adjectives and possessives agree with the gender, number, and case of the noun.
Budućnost (future) is feminine singular, so naš must become the feminine form naša to match:

masculine: naš prijatelj (our friend – m.)
feminine: naša budućnost (our future)
neuter: naše dijete (our child)

What exactly does ovisi o tome mean, and why do we need o tome?

Ovisi comes from ovisiti and means to depend.
The normal construction is ovisiti o + locative, literally to depend on something.
O tome is on that, where o = on/about and tome is that (in locative).
So naša budućnost ovisi o tome literally = our future depends on that.
The o tome points forward to what is about to be specified: kakve odluke donosimo danas….

What form is tome, and how is it related to to?

To is the basic neuter demonstrative pronoun (this/that).
Tome is its locative singular form, used after prepositions like o (about/on):

nominative: to (this/that)
locative: (o) tome (about/on that)
Here it is required by the preposition o, which always takes the locative case.

Why is it kakve odluke and not koje odluke? What does kakve add?

Both kakve and koje can translate as what / which, but there is a nuance:

kakve odluke = what kind of decisions, focusing on the quality, type, nature of the decisions.
koje odluke = which decisions, more like choosing from a known set.
In this philosophical/general sentence, kakve odluke (what kind of decisions) is more natural.
Grammatically, kakve is feminine plural accusative, agreeing with odluke (decisions).

Why is the verb donosimo used with odluke? Could I say pravimo odluke or radimo odluke instead?

In Croatian, the standard collocation is donositi odluke (to make decisions), literally to bring decisions.
Praviti and raditi mean to make / to do, but they are not used with odluke in standard language.
So:

donositi odluke = correct, idiomatic
praviti odluke / raditi odluke = sound wrong or very non‑native

What tense and aspect is donosimo, and why is that used here?

Donosimo is present tense, imperfective aspect of donositi.
Imperfective aspect is used for repeated, habitual, or general actions, so it fits the idea of the decisions we (generally) make today.
The perfective partner is donijeti (to make/bring once, completed):

Kad god donosimo odluke, trebamo razmisliti. – Whenever we make decisions, we should think.
Danas smo donijeli važne odluke. – Today we made (completed) important decisions.

Isn’t danas, u sadašnjosti redundant? Why say both today and in the present?

You could say just danas (today) or just u sadašnjosti (in the present), and the sentence would still be correct.
Here, danas, u sadašnjosti is stylistic and emphatic: it reinforces the contrast between present and future and sounds a bit more philosophical or formal.
It’s like saying in English: the decisions we make today, in the present.

Why is it u sadašnjosti and not something like u sadašnjost?

The preposition u (in) normally takes locative when it means location or time in something.
Sadašnjost (present) is feminine; its locative singular form is sadašnjosti.
So:

u školi (in school – locative)
u budućnosti (in the future – locative)
u sadašnjosti (in the present – locative)
U sadašnjost would be accusative and would suggest movement into the present, which is not intended here.

What case is odluke in, and why?

Odluke here is feminine plural accusative.
It’s the direct object of the verb donosimo: we make (what?) decisions.
The nominative plural would also be odluke, but you can see it’s accusative from its function in the sentence (object of donositi).

How flexible is the word order here? Could I say Naša budućnost ovisi o tome, kažu…?

Croatian word order is quite flexible because relations are shown by cases and agreement, not position.
You could say:

Kažu da naša budućnost ovisi o tome… (neutral, standard)
Naša budućnost ovisi o tome, kažu, kakve odluke… (more stylistic/emphatic; kažu becomes a comment)
Both are grammatical, but the given version is the most straightforward for learners, and puts Kažu up front like English They say that….

Is Kažu da naša budućnost ovisi o tome… formal, informal, or neutral in style?

The sentence is neutral to slightly formal.
The vocabulary (budućnost, sadašnjost, ovisi) and the structure sound natural in written language, essays, speeches, or more reflective conversation.
In casual talk, speakers might shorten or simplify it, but this version is perfectly natural and widely acceptable in both spoken and written Croatian.