Detektiv koristi mobilnu tarifu.

Because mobilnu tarifu is the direct object of the verb koristi.

• mobilna tarifa = nominative singular, the basic dictionary form
• mobilnu tarifu = accusative singular

In Croatian, a feminine singular noun and its adjective usually change form in the accusative:

• mobilna tarifa → mobilnu tarifu

So in this sentence:

• Detektiv = subject
• koristi = uses
• mobilnu tarifu = a mobile plan / mobile tariff

This is why both the adjective and the noun change.

It is in the accusative singular.

The verb koristiti usually takes a direct object, and direct objects are commonly in the accusative case.

Here is the pattern:

• nominative: mobilna tarifa
• accusative: mobilnu tarifu

This is a very common pattern with feminine nouns ending in -a and their adjectives.

Koristi is the 3rd person singular present tense of koristiti.

So:

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

In Detektiv koristi mobilnu tarifu, koristi means uses or sometimes is using, depending on context.

Koristiti is normally imperfective.

That means it describes an action as ongoing, repeated, or general, rather than as a single completed event.

So this sentence can mean things like:

• the detective uses a mobile plan
• the detective is using a mobile plan

The exact English translation depends on context, but Croatian present tense often covers both.

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

So Detektiv koristi mobilnu tarifu could mean:

• The detective uses a mobile plan
• A detective uses a mobile plan
• The detective is using the mobile plan

Context tells you which meaning is intended.

Yes, detektiv is a masculine noun in this sentence.

It is in the nominative singular, because it is the subject.

For a female detective, Croatian may use detektivka. In some contexts, detektiv may also be used more generally, but detektivka makes the female gender explicit.

So you may see:

• Detektiv koristi mobilnu tarifu.
• Detektivka koristi mobilnu tarifu.

Both mean the detective uses a mobile plan, but the second clearly refers to a woman.

Mobilna tarifa usually refers to a mobile phone plan, cell phone plan, or mobile tariff.

Depending on context, English might translate it as:

• mobile plan
• phone plan
• mobile tariff

In everyday English, mobile plan or phone plan is often the most natural translation.

Yes. Croatian word order is more flexible than English word order.

The neutral, straightforward order here is:

• Detektiv koristi mobilnu tarifu.

But other orders are possible for emphasis, for example:

• Mobilnu tarifu koristi detektiv.
• Mobilnu tarifu detektiv koristi.

These still mean roughly the same thing, but they can shift the focus or emphasis. The standard learner-friendly order is the original subject + verb + object.

You know mostly from the case endings.

• detektiv is in the nominative, which is the usual case for the subject
• tarifu is in the accusative, which is a common case for the direct object

So even if the word order changed, the endings would still help show who is doing the action and what is being used.

It can mean either, depending on context.

Croatian present tense often covers both:

• general / habitual: the detective uses a mobile plan
• right now / current situation: the detective is using a mobile plan

Without extra context, the sentence is slightly more natural as a general statement, but both readings are possible.

A simple approximate pronunciation for an English speaker is:

• de-TEK-tiv KO-ri-sti mo-BIL-nu ta-RI-fu

A few helpful points:

• r in Croatian is usually a tapped or rolled sound
• vowels are generally clear and pure, not heavily reduced
• every letter is pronounced quite consistently

So Croatian spelling is much more regular than English spelling, which helps a lot once you learn the sound system.