Na kruh mažem paštetu kad nemamo sira.

In Croatian, mazati nešto na nešto means to spread something onto/on something.

So:

• mažem paštetu na kruh = I spread pâté on bread
• na kruh literally means onto/on bread

With na, Croatian often uses the accusative when the idea is placement onto a surface. Here, the bread is the surface the pâté goes onto.

In everyday speech, na kruh is very natural in this context.

Because kruh is a masculine noun, and for many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: kruh = bread
• accusative: kruh = bread (as the object/goal after na here)

This is very common with masculine inanimate nouns in Croatian.

Because paštetu is the accusative singular, and it is the direct object of mažem.

You are spreading what?
→ paštetu

So:

• nominative: pašteta
• accusative: paštetu

Since pašteta is a feminine noun ending in -a, its accusative singular usually changes to -u.

Mažem is the 1st person singular present tense of mazati, meaning to spread or to smear.

So mažem = I spread / I am spreading, depending on context.

In this sentence, it most naturally has a habitual meaning:

• Na kruh mažem paštetu kad nemamo sira.
• I spread pâté on bread when we don’t have cheese.

So it is not necessarily describing one action happening right now. It can describe what the speaker عادة does in that situation.

Because the speaker is talking about an individual action inside a shared situation.

• mažem = I spread
• nemamo = we don’t have

So the idea is:

• I put pâté on bread
• when we don’t have cheese

This is perfectly natural. The speaker is probably talking about food available in the household or to the group.

Because after nemamo (we do not have), Croatian often uses the genitive for the thing that is absent or lacking.

So:

• nominative: sir = cheese
• genitive: sira

That is why you get:

• imamo sir = we have cheese
• nemamo sira = we don’t have cheese

This genitive after negation is very common and important in Croatian.

The standard and most expected form is nemamo sira.

That genitive form is the normal choice after negation in sentences like this, especially with an uncountable or indefinite noun such as cheese.

So for learners, the safe rule is:

• imamo sir
• nemamo sira

Using nemamo sir would usually sound nonstandard or much less natural in this context.

Kad here means when.

So:

• kad nemamo sira = when we don’t have cheese

This suggests a repeated or typical situation: whenever that happens, the speaker spreads pâté on bread.

You could compare:

• kad = when, in the sense of a real or usual situation
• ako = if, more conditional

So:

• Kad nemamo sira, mažem paštetu na kruh.
  = When we don’t have cheese, I spread pâté on bread.

• Ako ne budemo imali sira, mazat ću paštetu na kruh.
  = If we don’t have cheese, I’ll spread pâté on bread.

In your sentence, kad is the natural choice for a habitual meaning.

Croatian word order is much more flexible than English word order.

All of these can be natural depending on emphasis:

• Mažem paštetu na kruh.
• Na kruh mažem paštetu.

Starting with Na kruh puts a bit more focus on what I put it on. It may sound like the speaker is setting the scene with the bread first.

English usually has more fixed word order, but Croatian often moves parts of the sentence around for emphasis, rhythm, or topic.

Most likely it expresses a general habit or usual action.

Because of kad nemamo sira (when we don’t have cheese), the sentence sounds like:

• Whenever we don’t have cheese, I spread pâté on bread.

Croatian present tense often covers both:

• what is happening now
• what someone generally does

Context tells you which meaning is intended. Here the habitual reading is the most natural one.

Yes. Croatian does not have articles like the or a/an, so kruh can be interpreted in different ways depending on context.

It could mean:

• bread
• the bread
• some bread

In this sentence, English would usually just say on bread or on the bread, depending on the situation.

Croatian leaves that distinction to context.

Pašteta usually refers quite specifically to pâté or a pâté-like spread, often the kind sold in small tins or packages.

It is not a general word for all spreads.

For example:

• pašteta = pâté
• namaz = spread, spreadable topping

So in your sentence, paštetu really means pâté, not just any spread.

Yes. A very neutral version would be:

• Mažem paštetu na kruh kad nemamo sira.

This keeps the verb first and may be easier for beginners to process.

Your original sentence:

• Na kruh mažem paštetu kad nemamo sira.

is also correct, but it front-loads na kruh for emphasis or style. Both mean the same thing.