Na pregači je ostala mala mrlja od džema.

Because na is expressing a location here: on the apron. After na in the sense of location, Croatian uses the locative case.

• dictionary form: pregača = apron
• locative singular: pregači

So:

• na pregači = on the apron

If it meant motion onto the apron, Croatian would normally use the accusative instead:

• na pregaču = onto the apron

Because na usually means on, while u usually means in / inside.

So:

• na pregači = on the apron
• u pregači would sound like in the apron, which does not fit the meaning here

Since a stain is on the surface of the apron, na is the natural choice.

Je ostala is the perfect tense of the verb ostati, which means to remain, to stay, to be left.

Here it means something like:

• remained
• was left
• stayed

So je ostala is literally has remained, but in natural English it is often translated simply as remained or was left.

Because je is a clitic: a short, unstressed word that usually goes in the second position in the sentence or after the first full phrase.

In this sentence, the first phrase is Na pregači, so the clitic je comes right after it:

• Na pregači je ostala mala mrlja od džema.

That is very typical Croatian word order.

You can also find other word orders, for example:

• Mala mrlja od džema ostala je na pregači.

There, je comes after the first phrase Mala mrlja od džema.

Because both forms agree with mrlja, which is a feminine singular noun.

• mrlja = stain, feminine singular
• mala = small, feminine singular
• ostala = remained, feminine singular past participle

Croatian adjectives and past participles agree with the noun in gender, number, and often case.

If the subject were masculine, you would get masculine forms, for example:

• trag je ostao mali would use masculine forms

Yes. The subject is mala mrlja od džema.

English speakers often expect the subject to come before the verb, but Croatian word order is much more flexible. Here the sentence begins with the location Na pregači, and the subject comes later.

You can tell mrlja is the subject because:

• it is in the nominative
• the participle ostala agrees with it in feminine singular

So the sentence structure is basically:

• On the apron
  • was left/remained
    • a small jam stain

Džema is in the genitive singular because it follows the preposition od.

• od often means of, from, or made of / caused by
• džem = jam
• džema = genitive singular of džem

So:

• mrlja od džema = a stain from jam / a jam stain

In this context, od shows what caused the stain or what it came from.

Normally, mrlja od džema is the natural way to say a stain from jam.

Using od makes the relationship clear: the stain was caused by jam.

A bare noun-noun structure like English jam stain is generally not formed the same way in Croatian. Croatian usually prefers:

• mrlja od džema
• or sometimes an adjective if one exists, though here od džema is the normal solution

So English often compresses two nouns together, while Croatian often uses a preposition like od.

Yes. Croatian word order is flexible, although some versions sound more neutral than others.

This sentence could also be arranged as:

• Mala mrlja od džema ostala je na pregači.
• Na pregači je mala mrlja od džema ostala.
  This is possible, but less neutral and more marked.

The original version puts focus first on where the stain was:

• Na pregači = on the apron

So it is a natural choice if the location matters in the context.

Because Croatian has no articles.

English needs:

• a small stain
• the small stain

Croatian simply says:

• mala mrlja

Whether it means a or the depends on context. In this sentence, the most natural English translation is usually a small stain, but Croatian itself does not mark that difference with an article.

The main dictionary forms are:

• pregača = apron
• ostati = to remain, stay, be left
• mali = small
• mrlja = stain
• džem = jam

In the sentence, these appear in changed forms because of grammar:

• pregači = locative singular
• je ostala = perfect tense
• mala = feminine singular adjective form
• džema = genitive singular

It can suggest both, and that is why it is a useful Croatian verb.

In this sentence, ostala can be understood as:

• remained
• was left
• stayed there

So Na pregači je ostala mala mrlja od džema is not just about existence; it suggests that after something happened, the stain was still there.

That is why ostati works well here: it implies a result that continues.