Prostirka i vreća za spavanje su mokre.

Because the subject is made of two separate things joined by i (and): prostirka i vreća za spavanje.

Even though each noun is singular by itself, together they form a plural subject, so Croatian uses:

• su = are instead of je = is
• mokre = wet in the plural feminine form

So the grammar works like English: The mat and the sleeping bag are wet, not is wet.

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

• je = is for one thing
• su = are for more than one thing

Since the sentence talks about two items, Croatian needs su.

Croatian adjectives agree with the noun or nouns they describe in gender, number, and case.

Here:

• prostirka is feminine singular
• vreća is feminine singular
• together they become feminine plural

So the adjective must also be feminine plural:

• mokra = feminine singular
• mokre = feminine plural
• mokri = masculine plural

That is why mokre is the correct form here.

They are in the nominative singular.

Why? Because they are the subject of the sentence, and subjects are normally in the nominative.

So these are also the basic dictionary forms:

• prostirka
• vreća

It literally means for sleeping.

So:

• vreća = bag/sack
• za spavanje = for sleeping

Together, vreća za spavanje means sleeping bag.

This is a very common Croatian pattern:

• noun + za + verbal noun / noun to show purpose or function.

Both patterns can exist in Croatian, but vreća za spavanje is a very natural and common way to say sleeping bag.

Croatian often uses:

• noun + za + activity/purpose

So vreća za spavanje is literally a bag for sleeping.

You may also see adjective-based expressions in Croatian, but in this case vreća za spavanje is standard and very common.

Here spavanje is in the accusative singular after the preposition za.

A useful detail:

• spavanje is a neuter noun
• its nominative singular and accusative singular look the same

So even though the form does not change, the case here is still accusative because of za in this expression of purpose.

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

So:

• prostirka can mean a mat or the mat
• vreća za spavanje can mean a sleeping bag or the sleeping bag

The exact meaning depends on context, not on an article.

Because su is a clitic in Croatian. Clitics usually go near the second position in the sentence or after the first full phrase.

Here, the first phrase is the whole coordinated subject:

Prostirka i vreća za spavanje | su | mokre

So this word order sounds natural and standard.

English speakers often expect the verb to come directly after the first noun, but in Croatian clitic placement follows different rules.

Yes, Croatian word order is fairly flexible.

For example, you could also say:

• Mokre su prostirka i vreća za spavanje.

This puts more focus on mokre (wet).

But the original sentence:

• Prostirka i vreća za spavanje su mokre.

is a very natural, neutral way to say it.

Yes. Prostirka is a normal word for a mat, pad, or ground mat, depending on context.

In camping contexts, speakers may also use other words depending on region or exact meaning, such as words closer to pad or sleeping pad, but prostirka is perfectly understandable and natural here.

A rough English-friendly guide would be VRE-cha.

A few pronunciation notes:

• vr at the beginning is pronounced together; Croatian allows consonant clusters that may feel unusual to English speakers.
• ć is a soft ch/t sound, softer than English ch in chair.

So vreća is approximately VRE-cha, though the exact Croatian sound is a bit softer than that English approximation.