Kupila sam smeđu torbu za fakultet i sivu jaknu za jesenske dane.

In Croatian past tense, sam / si / je / smo / ste / su are clitics (short unstressed forms) and they normally cannot stand in the very first position in a sentence.

They usually go in second position (after the first stressed word or phrase).

• Kupila sam… – correct: kupila is the first stressed word, sam comes second.
• Ja sam kupila… – also correct: ja is first, sam is second, kupila is third.
• Sam kupila… – wrong in standard Croatian, because sam is first.

So kupila sam is a natural way to start the sentence when you don’t say ja.

In Croatian past tense, the main part of the verb (kupio / kupila / kupili…) is a kind of participle that agrees with the subject in gender and number.

• kupio sam – “I bought” (said by a man, masculine singular)
• kupila sam – “I bought” (said by a woman, feminine singular)

In your sentence, kupila sam… tells us that the speaker is female.

A man would say:

• Kupio sam smeđu torbu za fakultet i sivu jaknu za jesenske dane.

Croatian is a pro‑drop language: subject pronouns like ja (I), ti (you) are often left out when the meaning is clear from the verb form.

• Kupila sam… already shows:
  
  • sam = 1st person singular (“I”)
  • kupila = feminine speaker

So ja is optional here. Adding it just adds emphasis:

• Ja sam kupila smeđu torbu… – “I bought a brown bag…” (perhaps contrasting with someone else).

The basic form is:

• smeđa torba – “(the/a) brown bag” in the nominative singular (used for the subject).

Here, torbu is the direct object of kupila sam, so it must be in the accusative case (feminine singular). For most feminine nouns in -a, the accusative singular ending is -u:

• torba (nom. sg.) → torbu (acc. sg.)

The adjective must agree in case, gender and number with the noun:

• smeđa torba (nom. sg. fem.) → smeđu torbu (acc. sg. fem.)

So smeđu torbu is “brown bag” in the grammatical form needed as an object.

It’s because they describe different nouns in different forms:

1. smeđu torbu

   • torbu = feminine singular accusative
   • smeđu must match that → feminine singular accusative → -u

2. sivu jaknu

   • jaknu = feminine singular accusative
   • sivu matches → feminine singular accusative → -u

3. jesenske dane

   • dane = masculine plural accusative
   • jesenske matches → masculine plural accusative → -e

So the adjectives take different endings because torbu / jaknu are singular feminine, while dane is plural masculine.

Yes, they are all direct objects and all are in the accusative case, but Croatian nouns have different declension patterns:

• torba → torbu
  
  • feminine, singular, accusative ending -u

• jakna → jaknu

  • feminine, singular, accusative ending -u

• dan → dane

  • masculine, plural, accusative ending -e

• fakultet → fakultet

  • masculine inanimate, singular
  • for inanimate masculine nouns, accusative singular = nominative singular

So they are all accusative, but the spelling changes depend on gender (masc./fem.), number (sg./pl.), and the noun type.

The preposition za most often means “for” when talking about purpose or destination. It always takes the accusative case:

• za fakultet – “for college / university”
• za jesenske dane – “for autumn days”

It is repeated because it introduces two separate purpose phrases, each linked to a different object:

• smeđu torbu (for college)
• sivu jaknu (for autumn days)

If you wrote only one za and attached it to everything, the meaning could change (see the next question). Repeating za makes it perfectly clear which purpose belongs to which item.

You can say that, but the meaning becomes less specific and more ambiguous.

Kupila sam smeđu torbu i sivu jaknu za fakultet i jesenske dane.

Most speakers would understand it as: “I bought a brown bag and a grey jacket for college and for autumn days.” It sounds like both items are for both purposes together.

Your original sentence:

• Kupila sam smeđu torbu za fakultet i sivu jaknu za jesenske dane.

makes it clear that:

• the bag is for college,
• the jacket is for autumn days.

So the original is clearer and more precise about which purpose phrase goes with which noun.

Both are possible, but they have a slightly different feel:

• za jesen – literally “for autumn” (for the season in general)
• za jesenske dane – “for autumn days” (for the days during that season)

Za jesenske dane suggests many days throughout autumn when you will wear the jacket; it feels a bit more concrete and descriptive. It’s a common way in Croatian to talk about clothing and weather:

• jakna za zimske dane – a jacket for winter days
• cipele za kišne dane – shoes for rainy days

In Croatian, you normally do not put a comma before i (and), ni (nor), or ili (or) when they simply connect words or simple phrases in a list:

• Kupila sam knjigu, bilježnicu i olovku. – no comma before i

A comma might appear before i only in more complex cases (joining full clauses, adding a strong pause, etc.).

In your sentence, i is just joining two objects:

• smeđu torbu za fakultet
• sivu jaknu za jesenske dane

so no comma is used.

Not really. Croatian fakultet usually means:

• an institution where you study after high school → roughly “college / university”,
  e.g. Idem na fakultet. – “I go to college / university.”

It does not normally mean “the teaching staff” the way English faculty can.

So torba za fakultet is a bag for college / university, not “a bag for the professors.”

Yes, that word order is grammatically correct. Croatian has fairly flexible word order, especially in statements, and you can move parts of the sentence to change emphasis.

• Kupila sam smeđu torbu… – neutral: focus on what you did (you bought something).
• Smeđu torbu za fakultet i sivu jaknu za jesenske dane sam kupila.
  – puts strong emphasis on what exactly you bought, almost like:
  “Those are the things I bought.”

The meanings are the same, but the information focus and style change slightly.