U torbi imam pernicu, dvije kemijske i jedan crveni flomaster.

Because u meaning in normally takes the locative case when it shows location.

• dictionary form: torba = bag
• locative singular: torbi
• so u torbi = in the bag

If u shows movement toward somewhere, it usually takes the accusative instead, but here it is just location.

Croatian word order is more flexible than English word order. Putting u torbi first gives a natural topic-first structure, something like:

• As for what is in the bag, I have...
• In the bag, I have...

You could also hear:

• Imam pernicu, dvije kemijske i jedan crveni flomaster u torbi
• Imam u torbi pernicu, dvije kemijske i jedan crveni flomaster

But U torbi imam... sounds very natural.

Imam means I have.

It is the 1st person singular present form of imati = to have.

So:

• imam = I have
• imaš = you have
• ima = he, she, it has

In this sentence, Croatian uses I have to list what the speaker has in the bag.

Because pernica is the direct object of imam, so it goes into the accusative case.

• nominative: pernica
• accusative: pernicu

So:

• pernica = a pencil case, as a subject or dictionary form
• imam pernicu = I have a pencil case

This is a very common pattern in Croatian:

• Imam knjigu = I have a book
• Imam olovku = I have a pencil
• Imam pernicu = I have a pencil case

Pernica usually means a pencil case or pen case: the small case students keep pens, pencils, erasers, and similar things in.

So in school-related contexts, pernica is a very common word.

Because the understood noun is feminine.

Here, kemijske is short for kemijske olovke:

• kemijska olovka = ballpoint pen
• dvije kemijske olovke = two ballpoint pens

Since olovka is feminine, the number must also use the feminine form:

• dva for masculine and neuter
• dvije for feminine

So:

• dva flomastera = two markers
• dvije olovke = two pencils
• dvije kemijske = two pens

Because Croatian often leaves out a noun when it is obvious from context.

Here, kemijske is being used as a shortened form of kemijske olovke. This is very natural in everyday speech.

So:

• full form: dvije kemijske olovke
• shorter everyday form: dvije kemijske

A native speaker immediately understands that kemijske means pens here.

Literally, kemijska comes from kemijska olovka, which means ballpoint pen.

In everyday Croatian, people very often shorten this to just kemijska.

So:

• kemijska olovka = ballpoint pen
• kemijska = pen

The plural is:

• kemijske olovke = ballpoint pens
• kemijske = pens

This shortening is extremely common.

This phrase shows agreement between the number, adjective, and noun.

• flomaster is masculine singular
• jedan = one, masculine singular
• crveni = red, masculine singular
• flomaster = marker

So all the words match.

Also, because flomaster is a masculine inanimate noun, its accusative singular looks the same as its nominative singular. That is why you see:

• nominative: jedan crveni flomaster
• accusative: imam jedan crveni flomaster

The form does not change.

In Croatian, adjectives usually come before the noun they describe, just like in English.

So:

• crveni flomaster = red marker
• plava torba = blue bag
• mala pernica = small pencil case

The adjective also has to agree with the noun in gender, number, and case.

Yes, it could.

• jedan crveni flomaster = one red marker
• crveni flomaster = a red marker / red marker

Croatian has no articles like a or the, so jedan is only used when the speaker wants to emphasize the number one or make the list more explicit.

In a list like this, using jedan is very natural because the speaker is counting items:

• one pencil case
• two pens
• one red marker

This is the normal way to list items in Croatian.

• pernicu, dvije kemijske i jedan crveni flomaster

This works like English:

• a pencil case, two pens, and one red marker

Croatian usually uses commas between earlier items and i = and before the last one.

They are functioning as direct objects of imam, so they are in the accusative.

However, the forms do not all look obviously different:

• pernicu clearly shows accusative feminine singular
• jedan crveni flomaster looks like nominative, but for masculine inanimate nouns the accusative is the same as the nominative
• dvije kemijske also fits the object role, and the shortened phrase stands for dvije kemijske olovke

So even when the form looks unchanged, the grammatical role is still that of the accusative object.