Na kauču imamo novu navlaku koja se lako pere.

Because na takes different cases depending on whether it expresses location or movement.

• na kauču = on the couch / on the sofa → location, so Croatian uses the locative
• na kauč = onto the couch → movement toward a place, so Croatian uses the accusative

Here, the sentence describes where something is, not movement, so na kauču is correct.

Also note the form change:

• nominative: kauč
• locative: kauču

That -u ending is normal for many masculine nouns in the locative.

Because imamo takes a direct object, and direct objects are usually in the accusative.

The dictionary form is:

• nova navlaka = a new cover (nominative)

But after imamo (we have), it becomes:

• novu navlaku = accusative singular

Both the adjective and noun change because they must agree:

• nova → novu
• navlaka → navlaku

So:

• Imamo novu navlaku. = We have a new cover.

Navlaku is accusative singular feminine.

You can tell this because:

1. the verb imati (to have) normally takes a direct object
2. navlaka is a feminine noun ending in -a
3. many feminine nouns in -a change to -u in the accusative singular

So:

• nominative: navlaka
• accusative: navlaku

This is a very common pattern in Croatian.

This is a very common learner question.

Koja agrees with navlaku in gender and number:

• navlaka is feminine singular
• so the relative pronoun must also be feminine singular

But the case of koja is not determined by navlaku directly. Its case depends on its role inside the relative clause:

• koja se lako pere = which washes easily / which is easy to wash

Inside that clause, koja is the subject of pere se, so it is in the nominative.

That is why we get:

• navlaku = accusative, because it is the object of imamo
• koja = nominative, because it is the subject of se pere

If it were the object inside the relative clause, you would see koju instead.

Literally, it means something like:

• which washes itself easily
• or more naturally, which is easy to wash

In Croatian, se is often used in a way that sounds reflexive on the surface but is better understood in English as:

• a general statement
• a middle voice
• or something close to passive meaning

So lako se pere does not mean the cover literally washes itself. It means:

• it washes easily
• it can be washed easily
• it is easy to wash

This is a very common Croatian structure.

Croatian se is a clitic, which means it likes to stand in a special position in the sentence, usually near the beginning of its clause.

In the relative clause:

• koja se lako pere

the word se comes right after koja, which is very natural Croatian word order.

You may also hear or see:

• koja se pere lako

but koja se lako pere sounds more natural here because lako nicely modifies the whole verb phrase.

So the placement of se is mostly about Croatian clitic rules and natural rhythm.

Because Croatian usually does not need subject pronouns when the verb ending already shows who the subject is.

Here:

• imamo = we have

The ending -mo already tells you the subject is we.

So mi is optional. You would usually add it only for emphasis or contrast:

• Mi imamo novu navlaku... = We have a new cover...
  (emphasizing we, maybe in contrast with someone else)

In normal neutral sentences, leaving out the pronoun is the standard thing to do.

Yes. You could say:

• Imamo novu navlaku na kauču koja se lako pere.

or similar variants, depending on what you want to emphasize.

The original sentence:

• Na kauču imamo novu navlaku koja se lako pere.

puts na kauču first, which gives a slight focus to the location or sets the scene first.

Croatian word order is more flexible than English, but the different orders can shift emphasis.

Very roughly:

• Na kauču imamo... = As for the couch / On the couch, we have...
• Imamo novu navlaku... = starts more directly with we have

Both can be grammatical, but the original sounds natural and context-friendly.

In this sentence, na kauču most naturally means on the couch.

So the picture is:

• there is a couch
• on it, there is a new cover
• that cover is easy to wash

If you wanted to stress that the cover is for the couch, Croatian might use other phrasing depending on context, such as a structure with za or a different arrangement.

So here, the most straightforward reading is location: on the couch.

The basic dictionary form is koji. It means who/which/that as a relative pronoun or which as a question word, depending on context.

It changes for:

• gender
• number
• case

Here it appears as koja because it refers to a feminine singular noun.

Some nominative forms are:

• masculine singular: koji
• feminine singular: koja
• neuter singular: koje
• masculine personal plural: koji
• feminine plural: koje
• neuter plural: koja

In this sentence, koja = which/that, referring to navlaka.

Formally, pere is an active verb form:

• prati = to wash
• pere = washes / is washing

But when Croatian adds se in a sentence like this:

• lako se pere

the meaning becomes something like a middle voice or an impersonal passive-like expression.

So in English, the best translation is usually not active:

• not it washes itself easily

but rather:

• it washes easily
• it can be washed easily
• it is easy to wash

So the grammar is active + se, but the meaning is closer to English passive or middle-style wording.

The relative clause is:

• koja se lako pere

It describes the noun:

• novu navlaku

So the structure is:

• Na kauču = location
• imamo = main verb
• novu navlaku = direct object
• koja se lako pere = extra information about the cover

In English, this works just like:

• a new cover that is easy to wash
• a new cover which washes easily

So the relative clause tells you more about navlaku.