Kad dođeš do mosta, vidjet ćeš rijeku.

Yes. Kad means when, and it is a very common shorter form of kada.

So:

• kad = when
• kada = when (a bit fuller/more formal)

In everyday Croatian, kad is extremely common, and in this sentence it sounds completely natural.

Croatian often leaves out subject pronouns because the verb ending already shows who the subject is.

Here:

• dođeš = you come / you arrive
• vidjet ćeš = you will see

The ending -š tells you the subject is you (singular).

So Croatian does not need ti here. You could add ti for emphasis, but normally it is omitted.

This is a very common Croatian pattern.

After kad referring to the future, Croatian usually uses the present tense form of a perfective verb, not the future tense.

So:

• Kad dođeš do mosta... = When you get to the bridge...
• not usually Kad ćeš doći... in this kind of meaning

Even though dođeš looks like a present-tense form, here it refers to a future action because it is inside a time clause with kad.

This is different from English, but actually English does something similar:

• When you get to the bridge, you will see the river.

English also uses present get after when, not will get.

Dođeš is the 2nd person singular present form of doći, meaning to come / to arrive / to get to.

In this sentence, the most natural sense is:

• dođeš do mosta = you get to the bridge / you reach the bridge

So it is not just general come, but more specifically arrive at / reach that point.

The preposition do usually takes the genitive case.

Here:

• most = bridge (dictionary form, nominative)
• mosta = genitive singular

So:

• do mosta = to the bridge / as far as the bridge / up to the bridge

In this sentence, do mosta means to the bridge in the sense of reaching that point.

This is the standard way Croatian writes the future tense when the infinitive comes before the auxiliary.

The future is built with:

• the infinitive
• plus short forms of htjeti: ću, ćeš, će, ćemo, ćete, će

With many verbs, if the infinitive comes first, the final -i is dropped in writing:

• vidjeti → vidjet ćeš
• raditi → radit ću

So:

• vidjet ćeš = you will see

You may also see the order reversed:

• ćeš vidjeti

That is also grammatical, but vidjet ćeš is perfectly standard.

Because rijeku is the accusative singular, and the word is the direct object of vidjet ćeš.

Here:

• rijeka = river (nominative, dictionary form)
• rijeku = river in the accusative

Since you will see the river, the river is what is being seen, so Croatian uses the accusative:

• vidjet ćeš rijeku

This is very typical for feminine nouns ending in -a:

• nominative: rijeka
• accusative: rijeku

Because Kad dođeš do mosta is a subordinate clause, and it comes before the main clause.

So the structure is:

• Kad dođeš do mosta, = subordinate time clause
• vidjet ćeš rijeku. = main clause

Croatian normally separates that opening subordinate clause with a comma.

Yes. Croatian word order is more flexible than English, although some orders sound more natural than others.

The given sentence is the most neutral and natural:

• Kad dođeš do mosta, vidjet ćeš rijeku.

But you could also say things like:

• Rijeku ćeš vidjeti kad dođeš do mosta.
• Kad dođeš do mosta, rijeku ćeš vidjeti.

These versions shift the emphasis a little. The original sentence is a very normal default choice.

In this sentence, it is understood as a single completed event: once you reach the bridge, you will see the river.

So the meaning is naturally perfective-like: a one-time result, not an ongoing or repeated action.

That fits the whole sentence well:

• first you reach the bridge
• then you see the river

For a learner, the important practical point is simply that vidjet ćeš rijeku means you will see the river as a future event.

A few sounds may stand out to an English speaker:

• đ in dođeš is like a soft j/dj sound, roughly similar to the dg in bridge, but softer
• č in ćeš is a harder ch sound
• ć is also a ch-like sound, but softer than č

Also:

• dođeš is roughly DOH-jesh with a soft middle sound
• vidjet ćeš is roughly VEED-yet chesh

Exact pronunciation varies a bit by speaker and accent, but those approximations will help you get close.