Moja sestra koristi mobilno bankarstvo.

Because sestra is a feminine singular noun, and the possessive adjective moj must agree with it.

So:

• moj = my (masculine singular)
• moja = my (feminine singular)
• moje = my (neuter singular)

Since sestra means sister and is feminine, you say moja sestra.

Koristi is the 3rd person singular present tense of koristiti, meaning to use.

So:

• ja koristim = I use
• ti koristiš = you use
• on/ona koristi = he/she uses

In this sentence, the subject is moja sestra, so koristi means uses.

It can mean either, depending on context.

In Serbian, the present tense often covers both:

• she uses
• she is using

So Moja sestra koristi mobilno bankarstvo could mean:

• My sister uses mobile banking (general habit)
• My sister is using mobile banking (right now, if the context supports that)

Usually, without extra context, English learners should read it as a general statement: My sister uses mobile banking.

Because bankarstvo is a neuter singular noun, and the adjective mobilno must agree with it.

Agreement here is:

• mobilni = masculine
• mobilna = feminine
• mobilno = neuter

So:

• mobilni telefon = mobile phone
• mobilna aplikacija = mobile app
• mobilno bankarstvo = mobile banking

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

Koristiti takes a direct object, so mobilno bankarstvo is in the accusative case.

But:

• neuter singular inanimate nouns often have the same form in nominative and accusative

So:

• nominative: mobilno bankarstvo
• accusative: mobilno bankarstvo

The case is there grammatically, but there is no visible change in the form.

Because Serbian does not have articles like English a/an and the.

So Serbian simply says:

• Moja sestra koristi mobilno bankarstvo

English has to choose:

• My sister uses mobile banking
• My sister uses the mobile banking service
  depending on context

In Serbian, article meaning is usually understood from context.

You could, but it sounds more marked or poetic, not the most neutral everyday word order.

The normal, neutral order is:

• Moja sestra koristi mobilno bankarstvo.

Serbian word order is more flexible than English, but different orders can change emphasis.

For example:

• Moja sestra koristi mobilno bankarstvo. = neutral
• Mobilno bankarstvo koristi moja sestra. = emphasis on mobile banking
• Moja sestra mobilno bankarstvo koristi. = possible, but more stylistically marked

For learners, the standard version is best.

Yes, but it is usually unnecessary.

You could say:

• Moja sestra koristi mobilno bankarstvo.
• Ona koristi mobilno bankarstvo. = She uses mobile banking
• Moja sestra, ona koristi mobilno bankarstvo. = possible in special contexts, but not the neutral default

Serbian often omits subject pronouns because the verb form already shows the person and number, and the noun moja sestra already identifies the subject clearly.

Yes. Bankarstvo means banking.

So:

• mobilno bankarstvo = mobile banking
• internet bankarstvo = internet banking / online banking

In real life, people may also use terms influenced by English or by banking app language, but mobilno bankarstvo is a normal and correct phrase.

In Cyrillic, it is:

Моја сестра користи мобилно банкарство.

Serbian officially uses both scripts:

• Latin: Moja sestra koristi mobilno bankarstvo.
• Cyrillic: Моја сестра користи мобилно банкарство.

Learners should be able to recognize both.

A simple pronunciation guide is:

MO-ya SES-tra KO-ri-sti mo-BIL-no ban-KAR-stvo

A few helpful points:

• j is pronounced like English y in yes
• r is trilled or tapped
• č, ć, š, ž do not appear here, so the spelling is fairly straightforward
• Serbian spelling is very phonetic: words are usually pronounced much as they are written

So:

• Moja sounds like MO-ya
• sestra sounds like SES-tra
• koristi sounds like KO-ri-sti
• mobilno sounds like mo-BIL-no
• bankarstvo sounds like ban-KAR-stvo

Koristiti is imperfective.

That matters because imperfective verbs are commonly used for:

• habits
• repeated actions
• ongoing situations
• general facts

So in this sentence, koristi fits very naturally for a general statement like My sister uses mobile banking.

A perfective verb would not normally be used here for a simple habitual meaning.