Ona mu je namazala dvije kriške tosta prije škole.

Mu is the unstressed dative form of on (he), so here it means to him or for him.

In this sentence, it shows the person who benefited from the action:

• Ona mu je namazala... = She spread/prepared ... for him

Croatian very often uses these short pronoun forms instead of fuller phrases like njemu.

This is the past tense in Croatian, formed with:

• the auxiliary je (is/has in form, but used to build the past)
• the past participle namazala

So:

• je namazala = she spread / she prepared

This is the normal Croatian past-tense pattern:

• sam/si/je/smo/ste/su
  • participle

Examples:

• On je došao = He came
• Ona je došla = She came

Because the subject is feminine singular: ona.

In Croatian past tense, the participle agrees with the subject in gender and number:

• on je namazao = he spread
• ona je namazala = she spread
• oni su namazali = they spread
• one su namazale = they spread

So namazala tells you the subject is a woman/girl.

Because kriška is a feminine noun.

Croatian has different forms of two depending on gender:

• dva for masculine and neuter
• dvije for feminine

So:

• dva stola = two tables
• dva sela = two villages
• dvije kriške = two slices

Since kriška is feminine, dvije is correct.

After dva/dvije, tri, četiri, Croatian uses a special counting form. With many nouns, this looks like the genitive singular.

So:

• jedna kriška
• dvije kriške
• tri kriške
• četiri kriške

That is why kriška changes to kriške.

For learners, it is often easiest to remember this pattern:

• 2, 3, 4 + noun in the counting form
• 5 and above + genitive plural

Because kriška tosta means a slice of toast, and after a noun like kriška (slice), Croatian usually puts the second noun in the genitive.

So:

• kriška tosta = a slice of toast
• čaša vode = a glass of water
• komad kruha = a piece of bread

Here:

• dvije kriške tosta = two slices of toast

So tost changes to its genitive singular form tosta.

Here namazati means something like to spread something on bread/toast.

Depending on context, it could mean spreading:

• butter
• jam
• pâté
• chocolate spread
• etc.

Croatian often leaves that spread substance unstated if it is obvious from context. So namazala dvije kriške tosta naturally means she spread something on two slices of toast.

Also, namazati is usually a perfective verb, which means the action is seen as completed.

A related imperfective verb is:

• mazati = to be spreading / to spread in a general or repeated sense

Because the preposition prije (before) requires the genitive case.

So:

• škola = school
• škole = of school / before school after prije

That is why you get:

• prije škole = before school

Other examples:

• prije ručka = before lunch
• prije posla = before work
• prije sastanka = before the meeting

Usually it means before school / before going to school, not literally in front of the school building.

In everyday Croatian, prije škole is commonly used like English before school.

If you wanted to say in front of the school, you would use a different expression, such as:

• ispred škole = in front of the school

So context matters, but here the normal meaning is before school started / before leaving for school.

It could definitely be omitted.

Croatian often leaves out subject pronouns when they are already clear from context. So you could also say:

• Mu je namazala dvije kriške tosta prije škole.
• Namazala mu je dvije kriške tosta prije škole.

Including ona usually adds emphasis, contrast, or clarity:

• Ona mu je namazala... = She spread it for him

This can sound like:

• She did it (not someone else)
• or simply slightly more explicit

Yes, Croatian word order is fairly flexible, but the short unstressed words like mu and je usually follow special placement rules. They tend to appear near the beginning of the clause, in the second position area.

That is why mu je appears early.

This sentence could also appear as:

• Namazala mu je dvije kriške tosta prije škole.
• Prije škole mu je namazala dvije kriške tosta.
• Dvije kriške tosta mu je namazala prije škole.

These versions are all possible, but they shift the focus slightly.

One thing that is not natural is separating the little clitics in the wrong way. Croatian likes these short forms to stay in their usual clitic position.

Because both are clitics: short unstressed forms that cannot normally stand on their own.

Here:

• mu = dative pronoun clitic
• je = auxiliary clitic

Croatian often groups these short forms together:

• mu je
• joj je
• mi je
• ga je etc.

So Ona mu je namazala... is completely normal Croatian clitic placement.

No. The sentence already assumes the thing is toast or toasted bread, but namazala itself does not mean to toast.

It means to spread something on it.

So the sequence is more like:

1. there is toast
2. she spreads something on it

If you wanted to talk about making toast by heating bread, you would use a different verb, not namazati.