Learn Hardcore Croatian: Sutra ću ti pokazati svoje fotografije s mosta. - Tomorrow I will show you my photographs from the bridge.

ću is only used with ja (I), so Sutra ću… can only mean Tomorrow I will…
You can add ja for emphasis or contrast:
- Ja ću ti sutra pokazati svoje fotografije s mosta. = I will (as opposed to someone else) show you my photos from the bridge tomorrow.

So the I is “inside” the verb form rather than a separate word.

What does ću mean, and how does the future tense work here?

ću is the future auxiliary (a short form of the verb htjeti – to want), used to form Future I.

Pattern for future tense (Future I):
[person form of htjeti] + infinitive

ja ću pokazati – I will show
ti ćeš pokazati – you will show
on/ona će pokazati – he/she will show
mi ćemo pokazati – we will show
vi ćete pokazati – you (pl) will show
oni će pokazati – they will show

In your sentence:

ću = I will
pokazati = infinitive to show

So ću pokazati together mean will show.

You might also see a “split” form where the final -i of the infinitive drops, but only when ću comes after the verb:

Pokazat ću ti svoje fotografije.
(Here you cannot insert ti between pokazat and ću.)

What is the role of ti here, and how is it different from tebi or tvoje?

In Sutra ću ti pokazati svoje fotografije s mosta, ti is:

the indirect object = to you
dative singular of the pronoun ti (you, singular, informal)

So the structure is: I will show (what?) my photos (to whom?) to you.

Differences:

ti – unstressed dative form, normally used inside the sentence:
- Reći ću ti sutra. – I’ll tell you tomorrow.
tebi – stressed dative form, used for emphasis or contrast:
- Reći ću to tebi, a ne njemu. – I’ll tell it to you, not to him.
tvoje – possessive your (not used here):
- tvoje fotografije – your photos

So here ti does not mean your; it means to you.

What exactly does pokazati mean, and how is it different from pokazivati or pokazujem?

pokazati is the perfective infinitive “to show (once, as a complete act)”.

Aspect in Croatian:

pokazati – perfective (completed act, single event)
- Sutra ću ti pokazati… – Tomorrow I will show you… (once, to completion)
pokazivati – imperfective (ongoing/repeated)
- Sutra ću ti pokazivati svoje fotografije. – Tomorrow I’ll be showing you / will keep showing you my photos.
pokazujem – present tense, imperfective
- Sada ti pokazujem svoje fotografije. – I’m showing you my photos now.

For a simple future “I’ll show you… (once)”, pokazati is the natural choice.

Why is it svoje fotografije instead of moje fotografije?

Both are grammatically possible here, but svoje is more natural.

svoje is a reflexive possessive adjective. It refers back to the subject of the sentence (here: “I”).
moje is the normal possessive for my.

When the possessor is the subject, Croatian strongly prefers svoj (svoj, svoja, svoje, etc.):

Sutra ću ti pokazati svoje fotografije. – Tomorrow I’ll show you my (own) photos.

If you said:

Sutra ću ti pokazati moje fotografije.

it still means my photos, but it often sounds marked: either more emphatic or contrastive, like “my photos (not someone else’s)”. In neutral speech, reflexive svoje is standard when the subject owns the thing.

What form is fotografije in, and why does it look like that?

fotografije here is:

feminine plural,
accusative case,
functioning as the direct object of pokazati (to show what?).

Base noun: fotografija (a photo, photograph), feminine.

Declension (simplified):

Nominative singular: fotografija – a photo
Accusative singular: fotografiju – (I see) a photo
Nominative plural: fotografije – photos
Accusative plural: fotografije – (I see) photos

For inanimate feminine nouns like fotografija, nominative plural and accusative plural are the same: fotografije.

The adjective svoje agrees in gender, number, and case:

svoje fotografije – feminine, plural, accusative.

Why is it s mosta and not na mostu or od mosta?

Different prepositions express different relations:

s mosta = from (off) the bridge
- s
  - genitive expresses movement from a surface / from on top of something.
- Think: “down from the bridge”.
na mostu = on the bridge (location, not movement)
- na
  - locative expresses being on something:
    Stojim na mostu. – I am standing on the bridge.
od mosta = literally of/from the bridge, but usually means:
- origin or distance: 100 metara od mosta – 100 meters from the bridge
- possession/partitive: kraj od mosta – the end of the bridge (rare)

In your sentence, the photos were taken from the bridge, so s mosta is the natural choice: photos from (off) the bridge.

Why does most become mosta here, and where is “the” as in “the bridge”?

Case change
- Dictionary form: most – bridge (nominative singular)
- After s meaning from (off), the noun takes the genitive:
  - s mosta – from the bridge
So mosta is genitive singular of most.
No articles in Croatian
Croatian has no words for “a/an” or “the”. Context tells you whether mosta is a bridge or the bridge:
- s mosta can mean from a bridge or from the bridge, depending on what has been mentioned or is obvious in the situation.

So mosta combines the meaning “(the) bridge” and the genitive case needed after s.

What is the difference between s and sa? Could I say sa mosta?

s and sa are essentially the same preposition; sa is a longer form often used:

for easier pronunciation, especially before words starting with s, z, š, ž, č, ć, dž, đ or certain consonant clusters:
- sa stola – from the table
- sa ženom – with the woman
sometimes for emphasis or stylistic reasons.

With mosta, both s mosta and sa mosta are possible and understood as from the bridge.
In practice, s mosta is very common and sounds natural; sa mosta is also correct but a bit heavier-sounding.

Can the word order change, for example with sutra or the clitics ću and ti?

Yes, but with some rules, especially for clitics like ću and ti.

Basic word order in your sentence:
Sutra ću ti pokazati svoje fotografije s mosta.
Clitic placement
Clitics (ću, ti, mi, se, ga, je, ih…) usually go in the second position in the clause (after the first stressed word or phrase):
- Sutra ću ti pokazati…
- Ja ću ti sutra pokazati…
- Svoje fotografije ću ti sutra pokazati…
You cannot put ću at the very beginning:
✗ *Ću sutra ti pokazati… (wrong)
Possible variants (all correct, but with different emphasis):
- Sutra ću ti pokazati svoje fotografije s mosta. (neutral)
- Svoje fotografije s mosta ću ti sutra pokazati. (emphasis on my photos from the bridge)
- Pokazat ću ti sutra svoje fotografije s mosta. (verb-first, still neutral)

Word order is relatively flexible, but clitics like ću and ti must follow the second-position rule.

What is sutra grammatically, and does it change form?

sutra is a time adverb meaning tomorrow.

It does not change for case, gender, or number.
You can place it in different positions in the sentence:
- Sutra ću ti pokazati…
- Ja ću ti sutra pokazati…
- Pokazat ću ti svoje fotografije sutra.

Its function is simply to indicate when the action happens; grammatically it stays sutra in all contexts.