Krompir i luk su u kesi.

Su is the 3rd person plural form of biti (to be) in the present tense.

Even though krompir and luk each look singular by themselves, they are joined by i (and), so together they make a compound subject, and the verb must be plural:

• Krompir je u kesi. = one subject
• Luk je u kesi. = one subject
• Krompir i luk su u kesi. = two subjects, so plural

They are in the nominative case because they are the subject of the sentence.

This is the basic form you would find in a dictionary:

• krompir
• luk

In this sentence, they are the things that are in the bag, so nominative is the expected case.

Because after u meaning in for a location, Serbian uses the locative case.

The noun kesa changes like this:

• nominative: kesa
• locative: kesi

So:

• u kesi = in the bag

This is a very common Serbian pattern:

• u kući = in the house
• u školi = in the school
• u kesi = in the bag

Here it is locative.

The form kesi can look the same in both dative and locative singular, but the preposition tells you what it is doing here.

Because the sentence has u meaning in/inside a place, the case is locative:

• u kesi = in the bag

So the form is the same, but the function here is locative.

This is a very important distinction:

• u kesi = in the bag → location, so locative
• u kesu = into the bag → movement/direction, so accusative

Compare:

• Krompir i luk su u kesi. = They are already in the bag.
• Stavljam krompir i luk u kesu. = I am putting the potato and onion into the bag.

So with u, Serbian often contrasts:

• where? → locative
• where to? → accusative

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

That means a sentence like Krompir i luk su u kesi can be understood from context as something like:

• potato and onion are in a bag
• the potato and the onion are in the bag
• potatoes and onion are in the bag

English must choose an article, but Serbian usually leaves that to context.

If Serbian speakers want to be more specific, they can use other words, such as demonstratives:

• ta kesa = that bag
• taj krompir = that potato

Because Krompir i luk is one whole coordinated noun phrase: potato and onion.

In Serbian, short forms like su often go in the second position of the clause, but that means after the first whole unit, not necessarily after the first single word.

So:

• Krompir i luk su u kesi. is normal.

You do not split the coordinated subject like this:

• *Krompir su i luk u kesi — wrong

So the whole subject phrase stays together, and su comes after it.

Yes. Serbian word order is fairly flexible, though some versions sound more neutral than others.

The given sentence:

• Krompir i luk su u kesi.

is a normal, neutral way to say it.

You can also say:

• U kesi su krompir i luk.

This puts more focus on where they are.

Because Serbian uses case endings, word order can shift more than in English, but not every order sounds equally natural. The original sentence is a very standard choice.

Not always.

In Serbian, food nouns are often used in a more general or ingredient-like way, especially in everyday contexts like shopping, cooking, or describing what is in a bag.

So krompir i luk may refer to:

• one potato and one onion
• potato and onion as food items
• potatoes and onion, depending on context

This is especially natural with food vocabulary. If the speaker wants to be very exact, they can make it clearer:

• jedan krompir i jedan luk = one potato and one onion
• krompiri = potatoes

So the bare nouns can be somewhat broader in meaning than a word-for-word English interpretation might suggest.

Normally, no.

In standard Serbian, the present tense of biti (to be) is usually kept in sentences like this:

• Krompir i luk su u kesi.

Leaving out su would sound ungrammatical or at least very unnatural in ordinary standard Serbian.

This is worth noticing because some other Slavic languages often omit present-tense to be, but Serbian usually does not do that here.