Dno je debelo.

Dno is a noun, and here it is the subject of the sentence.

Grammatically, it is:

• singular
• neuter
• in the nominative case (the basic dictionary form, used for the subject)

That matters because the adjective debelo has to agree with dno in gender and number.

Because dno is a neuter noun.

In Croatian, adjectives must agree with the noun they describe:

• debeo = masculine singular
• debela = feminine singular
• debelo = neuter singular

So:

• stol je debeo would be masculine
• knjiga je debela would be feminine
• dno je debelo is neuter

This is one of the most important agreement patterns in Croatian.

Je is the 3rd person singular present tense form of the verb biti = to be.

So the structure is:

• dno = the bottom/base
• je = is
• debelo = thick

Croatian often uses biti in exactly the same way English uses to be in simple descriptive sentences:

• On je visok. = He is tall.
• Voda je hladna. = The water is cold.
• Dno je debelo. = The bottom is thick.

Croatian has no articles, so there is no direct equivalent of the or a/an.

That means dno can mean:

• the bottom
• a bottom
• sometimes just bottom, depending on context

The exact meaning is understood from the situation, not from an article.

Yes, dno is a neuter noun.

Its ending -o is a strong clue, because many Croatian neuter nouns end in:

• -o
• -e

For example:

• selo = village
• more = sea
• pismo = letter
• dno = bottom

This is not an absolute rule for every noun in the language, but it is a very common and useful pattern.

Yes. Dno usually means the bottom of something, and depending on context it can be understood as:

• the bottom of a container
• the base of an object
• the bottom layer
• the bottom of the sea, river, etc.

It does not usually mean floor in the everyday room/building sense. For that, Croatian more often uses words like pod.

So in a sentence like Dno je debelo, the natural interpretation is that the bottom/base of something is thick.

The basic order here is:

subject + verb + adjective

• Dno je debelo.

This is the most neutral and natural order.

Croatian word order is more flexible than English, so other orders are possible for emphasis, for example:

• Debelo je dno.
• Je li dno debelo? = Is the bottom thick?

But for a learner, Dno je debelo is the standard form to use.

In a normal present-tense statement like this, no. You should keep je.

Croatian does sometimes omit forms of to be in certain styles, headlines, or special expressions, but in regular standard speech:

• Dno je debelo is correct
• Dno debelo is not a normal complete sentence

So for learners, it is best to think: if English says is, Croatian usually needs je.

Here it is an adjective, not an adverb.

It is describing the noun dno, so it must agree with it in gender and number.

Compare:

• Dno je debelo. = The bottom is thick.
  → debelo = adjective, agreeing with dno
• Govori debelo.
  → here debelo could function differently depending on context, but in your sentence it is clearly adjectival

A useful test is this: if the word describes what the noun is like, it is functioning as an adjective.

It is in the nominative singular neuter, because it is part of the predicate describing the subject dno.

In Croatian, after biti (to be), predicate adjectives usually match the subject:

• dno = nominative singular neuter
• debelo = nominative singular neuter

So even though debelo comes after the verb, it still agrees with the subject.

You would make both the noun and adjective plural.

For example:

• Dna su debela. = The bottoms are thick.

Here:

• dna = plural of dno
• su = are
• debela = neuter plural adjective form

Notice that neuter plural adjectives often look the same as feminine singular:

• debela

So:

• knjiga je debela = the book is thick/fat
• dna su debela = the bottoms are thick

Same form, different grammar.