Moja sestra često kupuje sapun i peškire u prodavnici.

Because moja has to agree with sestra in gender, number, and case.

• sestra = feminine singular
• so the possessive adjective moj changes to moja

Compare:

• moj brat = my brother
• moja sestra = my sister
• moje dete = my child

In this sentence, moja sestra is the subject, so it is in the nominative case.

Serbian normally does not use articles like English a/an and the.

So:

• sestra can mean a sister, the sister, or just sister, depending on context
• sapun can mean soap, a bar of soap, or the soap, depending on context

Serbian speakers understand definiteness from context, word order, and the situation, not from articles.

Često is an adverb meaning often. It tells you how frequently the action happens.

So:

• kupuje = buys / is buying
• često kupuje = often buys

It modifies the verb, just like English often does.

Yes, it can move, but Moja sestra često kupuje... is a very natural, neutral word order.

Serbian word order is more flexible than English because cases show the grammatical role of nouns. Still, some orders sound more neutral than others.

This sentence uses a very normal pattern:

• subject: Moja sestra
• adverb: često
• verb: kupuje
• objects: sapun i peškire
• place: u prodavnici

Other orders are possible for emphasis, for example:

• U prodavnici moja sestra često kupuje sapun i peškire.
• Moja sestra kupuje sapun i peškire često — possible, but less natural in ordinary speech

So the given order is the safest one for learners.

Kupuje is:

• 3rd person singular
• present tense
• from the imperfective verb kupovati = to buy, to be buying, to buy regularly

It matches moja sestra because that is she.

So:

• ja kupujem = I buy / am buying
• ti kupuješ = you buy
• on/ona kupuje = he/she buys

Because the sentence includes često (often), the imperfective verb is the natural choice: it describes a repeated or habitual action.

It is in the accusative case, but for this kind of noun, the accusative singular looks the same as the nominative singular.

Here is what is happening:

• dictionary form: sapun
• direct object form here: sapun
• case: accusative singular

This happens because sapun is a masculine inanimate noun. In Serbian, masculine inanimate nouns often have:

• nominative singular = accusative singular

So even though the word looks unchanged, grammatically it is functioning as an accusative object.

Because peškire is the accusative plural form, used here as a direct object.

The basic forms are:

• singular: peškir = towel
• nominative plural: peškiri = towels
• accusative plural: peškire

Since kupuje is buying towels, the noun must be in the accusative plural:

• kupuje peškire = buys towels

So the sentence mixes:

• sapun = accusative singular
• peškire = accusative plural

That is completely normal.

Because case in Serbian is about grammatical function, not about all nouns having the same visible ending.

Both nouns are direct objects of kupuje, so both are in the accusative. But different nouns form the accusative differently.

• sapun → accusative singular looks the same as nominative
• peškir → accusative plural becomes peškire

So they are doing the same job in the sentence, even though the forms look different.

Because after u meaning in, when you are talking about location, Serbian uses the locative case.

The base noun is:

• prodavnica = store, shop

Its locative singular is:

• u prodavnici = in the store

So:

• u prodavnici = location
• not u prodavnica, because that would leave the noun in the wrong case

This is a very important Serbian pattern:

• u + locative = location, being somewhere
• u + accusative = motion into somewhere

So:

• u prodavnici = in the store
• u prodavnicu = into the store / to the store

In your sentence, the meaning is where the buying happens, so Serbian uses u prodavnici.

Examples:

• Moja sestra je u prodavnici. = My sister is in the store.
• Moja sestra ide u prodavnicu. = My sister is going to the store / into the store.

Yes. It sounds natural and neutral.

A very literal breakdown is:

• Moja sestra = subject
• često = frequency adverb
• kupuje = verb
• sapun i peškire = direct objects
• u prodavnici = place

So the sentence follows a very learner-friendly pattern:

Subject + adverb + verb + object + place

Because Serbian has case endings, word order can change more easily than in English, but this version is clear and standard.

The main special letters here are:

• č = roughly like ch in church, but firmer
• š = sh in ship
• j = y in yes

So roughly:

• moja ≈ MO-ya
• često ≈ CHES-to
• peškire ≈ PESH-kee-reh
• prodavnici ≈ pro-da-vnee-tsee

These are only rough English approximations, but they are helpful for getting started.

Yes. That is also grammatical.

You are just changing the order of the two objects:

• sapun i peškire
• peškire i sapun

Both are possible. The difference is mainly one of emphasis or what sounds more natural in context. The original sentence is perfectly fine, but Serbian does allow this kind of small reordering.