Krenut ćemo prema gradu čim završimo ručak.

This is the normal Croatian way of forming the future tense when the auxiliary ćemo comes after the infinitive.

The full infinitive is krenuti. But when ću / ćeš / će / ćemo / ćete comes directly after the infinitive, the final -i is usually dropped:

• krenuti
  • ćemo → krenut ćemo

Compare:

• Krenut ćemo prema gradu.
• Mi ćemo krenuti prema gradu.

Both mean the same thing. In the second pattern, because ćemo comes before the infinitive, the infinitive stays complete: krenuti.

It is the future tense: Future I in Croatian.

It is made from:

• the infinitive of the main verb: krenuti = to set off, to start moving, to head off
• the auxiliary htjeti in clitic form: ćemo = we will

So krenut ćemo means we will set off / we will head off.

Because the preposition prema requires the dative case.

The noun grad means city / town. Its dative singular form is gradu.

So:

• grad = city (basic form)
• gradu = to/toward the city

That is why the sentence has prema gradu.

Not exactly.

• prema gradu = toward the city, in the direction of the city
• u grad = to the city / into the city

So prema gradu focuses on direction, while u grad more clearly suggests the destination.

For example:

• Krenuli smo prema gradu. = We set off toward the city.
• Idemo u grad. = We’re going to the city.

In many real situations they can feel close, but they are not identical.

This is very common in Croatian.

After words like:

• kad = when
• čim = as soon as
• ako = if

Croatian often uses the present tense for actions that will happen in the future.

So:

• čim završimo ručak literally looks like as soon as we finish lunch
• but it refers to a future situation

This is normal Croatian grammar. English also does something similar:

• We’ll leave as soon as we finish lunch.

English does not say as soon as we will finish lunch, and Croatian works similarly here.

This is about aspect, which is very important in Croatian.

• završiti is perfective: to finish, to complete
• završavati is imperfective: to be finishing, to finish repeatedly/habitually, or to focus on the process

In this sentence, the meaning is a single completed action that happens before another action:

• first we finish lunch
• then we set off

Because the action must be completed, Croatian uses the perfective verb: završimo.

Ručak is the direct object of završimo, so it is in the accusative case.

However, ručak is a masculine inanimate noun, and in the singular, masculine inanimate nouns often have the same form in nominative and accusative.

So:

• nominative: ručak
• accusative: ručak

That is why it looks unchanged.

Compare with a feminine noun, where the change is more obvious:

• večera = dinner
• završimo večeru = we finish dinner

Čim means as soon as.

It is more specific than kad, which usually means when.

So:

• čim završimo ručak = as soon as we finish lunch
• kad završimo ručak = when we finish lunch

In many contexts both are possible, but čim emphasizes that the second action happens immediately after the first one.

Yes. Croatian word order is fairly flexible, although some orders sound more neutral than others.

The original sentence is natural and neutral:

• Krenut ćemo prema gradu čim završimo ručak.

You could also say:

• Čim završimo ručak, krenut ćemo prema gradu.

This is also very natural and may sound slightly more structured because it puts the time condition first.

What you must watch carefully is the position of short clitics like ćemo. They usually come in the second position of their clause, so not every rearrangement is possible.

Not exactly.

• ići = to go
• krenuti = to set off, to start going, to head out

So krenut ćemo emphasizes the start of movement.

Compare:

• Ići ćemo prema gradu. = We will go toward the city.
• Krenut ćemo prema gradu. = We will set off toward the city.

The second one sounds more like we’ll get going.

This sentence has both sounds:

• č in čim and ručak
• ć in ćemo

A simple learner-friendly approximation is:

• č = like ch in church
• ć = a softer sound, somewhere between English t
  • y and a very soft ch

So roughly:

• čim ≈ cheem
• ćemo ≈ something like tyemo, but softer and smoother than that spelling suggests

Many English speakers hear č more easily than ć at first. That is normal.

Because Croatian does not have articles like a and the.

So Croatian simply says:

• prema gradu = toward the city / toward a city
• ručak = lunch / the lunch

The exact meaning depends on context.

In this sentence, English naturally uses the city only if that city is already understood from context. Croatian leaves that to context rather than marking it with an article.