Papuče su ispod kreveta.

Because papuče is the plural form of papuča, meaning slipper. So the sentence is talking about slippers, not just one slipper.

• singular: papuča
• plural: papuče

That is also why the verb is plural: su = are.

Su is the 3rd person plural present tense of the verb biti = to be.

So:

• ja sam = I am
• ti si = you are
• on/ona/ono je = he/she/it is
• mi smo = we are
• vi ste = you are
• oni/one/ona su = they are

Since papuče is plural, Croatian uses su.

Because the preposition ispod requires the genitive case.

The basic noun is:

• krevet = bed

Its genitive singular form is:

• kreveta

So:

• ispod kreveta = under the bed / below the bed

This is a very common thing in Croatian: certain prepositions require a specific case, and ispod takes the genitive.

Yes, ispod normally takes the genitive.

Examples:

• ispod stola = under the table
• ispod mosta = under the bridge
• ispod prozora = below the window

So once you learn ispod + genitive, you can reuse that pattern with many nouns.

Yes. Papuče su pod krevetom is also natural and very common.

There is a small nuance:

• pod + instrumental often means under
• ispod + genitive often means below / underneath

In many everyday situations, both can work:

• pod krevetom
• ispod kreveta

Both can mean under the bed.

Because Croatian has no articles like English a / an / the.

So krevet can mean:

• a bed
• the bed

The exact meaning depends on context.

In ispod kreveta, English usually translates it as under the bed, but Croatian does not need a separate word for the.

No, Croatian word order is fairly flexible.

Papuče su ispod kreveta is a neutral, natural order. But other orders are possible, depending on emphasis:

• Papuče su ispod kreveta. = neutral
• Ispod kreveta su papuče. = emphasizes location
• Ispod kreveta papuče su. = possible, but less natural in ordinary speech

So the sentence can move around more than in English, but the most normal version here is Papuče su ispod kreveta.

Normally, in a full standard sentence like this, su should stay.

• Papuče su ispod kreveta. = correct
• Papuče ispod kreveta. = incomplete in standard usage

Croatian sometimes drops elements in casual speech when the meaning is obvious, but for learners, it is best to keep su here.

A rough English-friendly pronunciation is:

PA-poo-cheh soo EES-pod KREH-veh-tah

A few useful sound notes:

• č sounds like ch in church
• c sounds like ts in cats
• j usually sounds like English y
• Croatian vowels are generally clear and short: a, e, i, o, u

So:

• papuče ≈ PA-poo-cheh
• su ≈ soo
• ispod ≈ EES-pod
• kreveta ≈ KREH-veh-tah

Here, papuče is the subject of the sentence, so it is in the nominative plural.

You can think of the structure like this:

• papuče = subject
• su = verb
• ispod kreveta = location phrase

So the sentence literally follows the pattern:

slippers + are + under the bed

It describes location.

The slippers are already there; they are under the bed.

There is no movement meaning such as going under the bed or putting them under the bed. It is simply stating where they are.