Odvodno crijevo je dugo.

Because dugo has to agree with crijevo, which is a neuter singular noun.

The adjective dug changes form depending on the noun:

• dug = masculine singular
• duga = feminine singular
• dugo = neuter singular

So:

• kabel je dug = the cable is long
• cijev je duga = the pipe is long
• crijevo je dugo = the hose is long

Here, crijevo is neuter, so dugo is the correct form.

Odvodno is an adjective modifying crijevo.

Its dictionary form is odvodni, meaning something like:

• drain
• drainage
• outlet

Because it describes crijevo, it also has to agree with it in gender, number, and case. Since crijevo is neuter singular nominative, odvodni becomes odvodno.

So odvodno crijevo means drain hose or drainage hose.

It is in the nominative singular.

Why? Because it is the subject of the sentence — the thing being described:

• Odvodno crijevo = the drain hose
• je dugo = is long

The sentence is telling us something about the hose, so the noun stays in the nominative.

Je is the 3rd person singular present tense of biti, which means to be.

So here it means is.

• je = is

This is a very common pattern in Croatian:

• X je Y = X is Y

For example:

• Voda je hladna. = The water is cold.
• Auto je novo. = The car is new.
• Odvodno crijevo je dugo. = The drain hose is long.

Because Croatian does not use articles like a, an, or the.

So crijevo can mean:

• a hose
• the hose

Which one is meant depends on the context.

That means Odvodno crijevo je dugo could be understood as:

• The drain hose is long
• or A drain hose is long

In real use, the situation usually makes the meaning clear.

Here it is an adjective, not an adverb.

It is describing the noun crijevo, and it agrees with that noun in form.

Compare:

• Crijevo je dugo.
  Here dugo = long and describes crijevo

But:

• Dugo čekam.
  Here dugo is an adverb meaning for a long time

So the same form can have different functions, but in this sentence it is clearly an adjective because it matches the neuter noun crijevo.

Yes, crijevo is a normal word for a flexible hose/tube.

It can also mean intestine/gut in other contexts, so context matters.

A useful comparison is:

• crijevo = hose, flexible tubing
• cijev = pipe, tube, usually more rigid

So for something like a washing machine drain hose, odvodno crijevo is very natural.

Yes, Croatian word order is more flexible than English, but the given order is the most neutral and natural:

• Odvodno crijevo je dugo.

This is the straightforward subject + verb + complement order.

You may sometimes see other orders for emphasis, for example:

• Dugo je odvodno crijevo.

That would put more focus on dugo, but it is more marked. For a learner, the original sentence is the best basic pattern to remember.

Normally, in a full standard sentence like this, no.

Croatian usually keeps the present tense of biti in this kind of sentence:

• Crijevo je dugo.

Leaving it out would sound incomplete in normal standard usage.

You might see omission in:

• headlines
• labels
• notes
• very informal fragments

But as a normal sentence, je should stay.

The dictionary forms are:

• odvodni = drain, drainage
• crijevo = hose / tube
• biti = to be
• dug = long

In the sentence, some of those forms change:

• odvodni → odvodno to match neuter crijevo
• dug → dugo to match neuter crijevo
• biti → je because it means is

So this sentence is a good example of how Croatian adjectives change to agree with the noun they describe.