Kad završim posao, svratit ću u mesnicu po meso za večeru.

Because after kad (when), Croatian often uses the present tense of a perfective verb to refer to a future completed action.

• završiti = perfective to finish
• završim = I finish / I will have finished in this kind of clause

So:

• Kad završim posao = When I finish work / Once I’ve finished work

This is very natural in Croatian. English usually uses a present form too in this kind of time clause: When I finish work..., not When I will finish work...

Kad means when.

In this sentence, it introduces a time clause:

• Kad završim posao = When I finish work

Depending on context, it can also feel like once:

• Once I finish work, I’ll stop by...

So here it marks the moment after which the next action will happen.

Posao is in the accusative singular because it is the direct object of završim.

• nominative: posao = work / job
• accusative: posao

For this noun, nominative and accusative look the same, so there is no visible ending change.

In:

• završim posao

the meaning is finish work or finish the job/work.

Svratit ću means I will stop by or I’ll drop in.

The verb is svratiti. In the future tense, Croatian often uses:

• the infinitive
• plus the clitic form of htjeti (ću, ćeš, će...)

With verbs ending in -ti, the final -i drops when the clitic follows:

• svratiti + ću → svratit ću

This is standard:

• doći ću
• kupit ću
• svratit ću

So svratit ću is just the normal future form of svratiti.

Yes. Both are possible, but the word order changes the form slightly.

You can say:

• Svratit ću u mesnicu...
• Ću svratiti u mesnicu... → this is not correct at the beginning of a sentence
• Ja ću svratiti u mesnicu... = correct
• Kad završim posao, svratit ću... = correct
• Kad završim posao, ja ću svratiti... = also correct

The clitic ću normally cannot stand in the first position of its clause, so if it comes after the verb, you get svratit ću. If something else comes before it, you can have ću + infinitive, for example:

• Ja ću svratiti
• Sutra ću svratiti

Not exactly.

Svratiti means something like:

• to stop by
• to drop in
• to make a quick stop
• sometimes to swing by

It suggests a brief or secondary stop, not just general going.

So:

• Idem u mesnicu = I’m going to the butcher’s
• Svratit ću u mesnicu = I’ll stop by the butcher’s

The second one implies it’s a short stop on the way or as part of a larger plan.

Because u can take different cases depending on meaning:

• u + accusative = movement into / to
• u + locative = location in / at

Here there is movement toward the butcher’s shop:

• svratit ću u mesnicu = I’ll stop by / go into the butcher’s

So mesnicu is accusative singular.

Compare:

• U mesnici je gužva. = It’s crowded in the butcher’s.
  Here mesnici is locative because it expresses location.

Mesnica means butcher’s shop or butcher shop.

It refers to the place, not the person.

• mesar = butcher (the person)
• mesnica = butcher’s shop

So:

• svratit ću u mesnicu = I’ll stop by the butcher’s shop

In this sentence, po + accusative means to get / for / in search of something.

So:

• ići po kruh = to go get bread
• svratiti po mlijeko = to stop by for milk
• u mesnicu po meso = to the butcher’s to get meat

Here:

• po = for / to pick up
• meso = accusative singular, though it looks the same as nominative

This is a very common Croatian pattern when someone goes somewhere in order to fetch or buy something.

Meso is in the accusative singular because it follows po in the sense of to get / fetch.

For this neuter noun, nominative and accusative singular are the same:

• nominative: meso
• accusative: meso

So although the form does not change, the case here is accusative.

Za večeru means for dinner.

Here za + accusative expresses purpose or intended use:

• kava za goste = coffee for the guests
• nešto za doručak = something for breakfast
• meso za večeru = meat for dinner

So the idea is:

• I’ll stop by the butcher’s to get meat for dinner.

Because Kad završim posao is a subordinate time clause placed before the main clause.

Croatian normally separates that kind of clause with a comma:

• Kad završim posao, svratit ću u mesnicu...

This is similar to English:

• When I finish work, I’ll stop by the butcher’s...

If the main clause came first, a comma would generally still be used before the subordinate clause:

• Svratit ću u mesnicu po meso za večeru kad završim posao.

In that version, the comma is often omitted because the time clause comes at the end and is tightly connected, but punctuation can vary with style.

Yes, both verbs express completed, single actions here.

• završiti is perfective: to finish
• svratiti is perfective: to stop by once / make a brief stop

That fits the meaning of the sentence well:

1. first, the speaker will finish work
2. then, the speaker will stop by the butcher’s

Croatian often uses perfective verbs for one-time future actions like this.

Yes. Posao can mean:

• work
• job
• task
• piece of work

So the exact translation depends on context.

In everyday speech, završiti posao often means:

• finish work
• be done with work

But in another context it could definitely mean finish the task/job.

Yes, it sounds natural and idiomatic.

Especially natural are these parts:

• Kad završim posao = common way to say when I finish work
• svratit ću = very natural for I’ll stop by
• u mesnicu po meso = standard Croatian structure for going somewhere to get something
• za večeru = normal way to say for dinner

So this is the kind of sentence a native speaker could easily say in everyday conversation.