Za ručak danas kuham juhu od leće, celera i mrkve.

Za ručak means for lunch. Starting the sentence with it sets the context first: we are talking about lunch.

Croatian word order is fairly flexible, so this is a very natural way to begin. It is similar to saying:

For lunch today, I’m making...

You could also say:

Danas za ručak kuham juhu...

That would still be correct, just with slightly different emphasis.

Literally, za ručak means for lunch.

The preposition za is often used for:

• purpose
• intended recipient
• a planned time/occasion

So here it means the soup is being made for lunch.

A few similar examples:

• za večeru = for dinner
• za doručak = for breakfast

So Za ručak danas kuham... is a very natural Croatian way to say I’m cooking ... for lunch today.

Because Croatian often leaves out subject pronouns when they are already clear from the verb ending.

kuham means I cook / I am cooking.
The ending -am shows that the subject is I.

So:

• kuham = I cook / I’m cooking
• ja kuham = I cook / I’m cooking, but with extra emphasis on I

You would use ja only if you want contrast or emphasis, for example:

• Ja kuham, a on pere suđe. = I’m cooking, and he is washing the dishes.

It can mean both.

Croatian present tense often covers both:

• I cook
• I am cooking

The exact meaning depends on context.

In this sentence, kuham most naturally means:

• I’m cooking or
• I’m making

Because the sentence is about today’s lunch, it sounds like a current plan or current activity.

So the whole sentence can sound like:

• For lunch today, I’m making lentil, celery, and carrot soup.

Because juhu is the accusative singular, and it is the direct object of kuham.

The basic dictionary form is:

• juha = soup

But after a verb like kuhati when soup is the thing being cooked, Croatian uses the accusative:

• Kuham juhu. = I’m cooking soup.

This happens because juha is a feminine noun ending in -a:

• nominative: juha
• accusative: juhu

This is a very common pattern:

• mrkva → mrkvu
• kava → kavu
• salata → salatu

Because od is commonly used to mean made from / of when talking about ingredients.

So:

• juha od leće = soup made from lentils
• juha od rajčice = tomato soup
• pire od krumpira = mashed potatoes, literally purée of potatoes

After od, Croatian uses the genitive case. That is why the nouns change form:

• leća → leće
• celer → celera
• mrkva → mrkve

So od leće, celera i mrkve means: made from lentils, celery, and carrots.

There are several useful case examples here:

• ručak in za ručak is in the accusative
• juhu is in the accusative because it is the direct object
• leće, celera i mrkve are in the genitive after od

So the sentence is a nice example of how prepositions and verbs affect case:

• za often takes the accusative
• od takes the genitive
• the direct object of the verb here is also accusative

That is just a difference between the two languages.

In English, we usually say lentils in the plural when talking about the ingredient.
In Croatian, leća is commonly used as a singular mass/collective noun for the food itself.

So:

• juha od leće = lentil soup

Even though English prefers plural lentils, Croatian naturally uses leća.

This is similar to how languages sometimes package ingredients differently as countable or uncountable nouns.

Yes. Croatian word order is more flexible than English word order.

This sentence:

• Za ručak danas kuham juhu od leće, celera i mrkve.

Could also be:

• Danas za ručak kuham juhu od leće, celera i mrkve.
• Kuham danas za ručak juhu od leće, celera i mrkve.

These all mean basically the same thing, but the emphasis changes slightly.

The original version sounds natural because it puts the meal context first:

• For lunch today...

Because Croatian does not have articles like English a/an/the.

So juhu can mean:

• soup
• a soup
• the soup

The exact meaning comes from context.

In this sentence, English would normally say just soup, without an article:

• For lunch today, I’m making lentil, celery, and carrot soup.

But Croatian does not need a separate word for that.

Mostly yes for a simple list.

In leće, celera i mrkve, the commas separate items in a list:

• lentils
• celery
• and carrots

Croatian usually does not use a comma before i in a simple list, just like standard British English and often like standard Croatian punctuation:

• A, B i C

So this punctuation is completely normal.

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

• kuhati is imperfective
• skuhati is perfective

kuham from kuhati focuses on the process or ongoing action:

• I cook / I’m cooking

That is why it fits well here.

If you used the perfective verb, the meaning would shift toward completion:

• Danas ću skuhati juhu... = Today I will cook / make the soup
  (with the idea of finishing it)

So kuham is a natural choice when describing what you are making for lunch.

A few parts may look difficult to an English speaker:

• ručak: the č sounds roughly like ch in church
• leće: ć is softer than č, though many learners just approximate it at first
• mrkve: this can feel awkward because of the consonant cluster mrkv-
• juhu: pronounced roughly YOO-hoo
• kuham: the h is clearly pronounced, unlike in many English words

Also remember:

• j in Croatian sounds like English y
• c sounds like ts
• č sounds like ch
• ć is a softer sound, somewhat between ty and ch for many learners

A rough learner-friendly pronunciation of the full sentence could be:

Za ROO-chak DAH-nas KOO-ham YOO-hoo od LEH-che, TSEH-leh-ra ee MURK-veh.

That is only approximate, but it helps as a starting point.