Namaži mi jednu krišku kruha maslacem, a drugu džemom.

Namaži is the imperative form of the verb namazati, meaning spread, put on by spreading, or smear on.

Here it is a command:

• Namaži mi... = Spread ... for me

A useful point for learners is that namazati is a perfective verb. In commands, perfective verbs often mean do it once / complete the action. So namaži sounds like:

• Spread it
• Put some on

If you used the imperfective verb mazati, the imperative maži would sound more like an ongoing action, or repeated spreading, and is less natural here for a simple one-time request.

Mi means to me / for me.

It is the short dative form of ja (I), and in this sentence it shows the person for whose benefit the action is done:

• Namaži mi jednu krišku kruha...
• Spread me a slice of bread...
• more natural English: Spread a slice of bread for me...

This is very common in Croatian. The dative often marks the person affected by or benefiting from the action.

Compare:

• Daj mi vodu. = Give me water.
• Kupi mi kruh. = Buy me bread.

Because jednu krišku is in the accusative case, which is used here for the direct object of the verb namaži.

The basic nominative forms are:

• jedna kriška = one slice

But after the verb, as the object, they change to accusative:

• jednu krišku

Both words are feminine singular and agree with each other:

• nominative: jedna kriška
• accusative: jednu krišku

The same thing happens with drugu later in the sentence:

• druga → nominative
• drugu → accusative

Drugu means the other one / another one, and it refers back to krišku.

So the full meaning is:

• jednu krišku kruha maslacem, a drugu džemom
• one slice of bread with butter, and the other (slice) with jam

Croatian often leaves out a noun when it is already understood from context. So drugu here really means:

• drugu krišku

This is very natural and common.

In krišku kruha, the word kruha is in the genitive singular because it follows a noun of quantity/part:

• kriška = slice
• kriška kruha = a slice of bread

This is similar to English a slice of bread, where bread comes after slice of.

So:

• kruh = bread
• kruha = of bread

Other similar examples:

• čaša vode = a glass of water
• komad sira = a piece of cheese
• vrećica šećera = a bag of sugar

Maslacem and džemom are in the instrumental case.

They answer the idea with what? / using what?

• maslacem = with butter
• džemom = with jam

After verbs like namazati, Croatian often uses the instrumental without a preposition to show the substance spread onto something.

So:

• Namaži kruh maslacem.
• Spread the bread with butter.

That is why you do not need s here.

Compare:

• s maslacem usually means with butter in the sense of accompaniment
• maslacem here means using butter as the spread

Here a has a contrastive meaning: and, while, or whereas.

The sentence is contrasting two different slices and two different toppings:

• one slice with butter,
• the other with jam.

So a is better than i because it highlights the difference between the two parts.

• ... maslacem, a drugu džemom.
• ... with butter, and the other with jam.

If you used i, it would sound more like simple addition and less like a contrast between the two slices.

Yes, more or less. Word for word, it is close to that.

A very literal breakdown is:

• Namaži = spread
• mi = to me / for me
• jednu krišku kruha = one slice of bread
• maslacem = with butter
• a = and / whereas
• drugu = the other one
• džemom = with jam

Natural English would usually be:

• Spread one slice of bread with butter for me, and the other with jam. or
• Butter one slice of bread for me, and put jam on the other.

The word order is somewhat flexible, but mi is a clitic, so it usually appears in second position in the clause.

That is why:

• Namaži mi jednu krišku kruha maslacem...

sounds natural.

You usually would not say:

• Namaži jednu krišku kruha mi...

because the clitic mi does not normally go there.

Croatian clitics like mi, ti, ga, je, se tend to come early in the sentence, often after the first stressed word or phrase.

So for a learner, the safest pattern is:

• Imperative + clitic + rest
• Daj mi...
• Reci mu...
• Namaži mi...

Exactly. Jednu ... drugu is a very common pattern meaning:

• one ... the other
• one ... another

In this sentence:

• jednu krišku = one slice
• drugu = the other one / the second one

Because there are two slices in view, English often prefers the other, while Croatian naturally uses drugu.

This pair is very common:

• Jedan je moj, a drugi tvoj.
• One is mine, and the other is yours.

You could, but it means something different.

• Namaži mi kruh maslacem i džemom = Spread my bread with butter and jam This sounds like both butter and jam go on the same bread, or at least it does not clearly separate them onto different slices.

The original sentence clearly means:

• butter on one slice,
• jam on the other slice.

So jednu ... a drugu ... is important if you want that exact meaning.

Yes, it sounds natural.

It is a normal way to ask someone to prepare bread for you, especially at breakfast or for a snack.

A Croatian speaker might also say similar things like:

• Namaži mi jednu šnitu kruha maslacem, a drugu džemom.
• Daj mi jednu s maslacem, a drugu s džemom.

But your sentence is perfectly normal and grammatically correct.

A few parts may be tricky for English speakers:

• Namaži: roughly NA-ma-zhi
  • ž sounds like the s in measure
• krišku: roughly KREESH-koo
  • š sounds like sh
• džemom: roughly JEM-om
  • dž sounds like the j in jam
• kruha: roughly KROO-ha

Also note that Croatian spelling is very regular, so once you know the sound values of letters like š, ž, dž, pronunciation becomes much easier.