Mobilna tarifa je skupa, stoga biram drugi paket.

Because tarifa is a feminine singular noun, and Croatian adjectives must agree with the noun in gender, number, and case.

So:

• skup = masculine singular
• skupa = feminine singular
• skupo = neuter singular

Since the sentence has mobilna tarifa, the correct form is skupa.

Here it means expensive.

That can be confusing, because skupa can also mean together in some contexts. But in this sentence, it is clearly the feminine form of the adjective skup = expensive.

The structure tarifa je skupa means the tariff is expensive, so this is definitely the adjective, not the adverb together.

Stoga means therefore, thus, or for that reason.

It is correct and natural, but it sounds a bit more formal or written than some everyday alternatives, such as:

• zato
• pa

So this sentence sounds slightly more polished or formal than very casual speech.

Croatian often drops subject pronouns when they are already clear from the verb ending.

Biram already means I choose / I am choosing, because the ending -m shows 1st person singular.

So:

• biram = I choose
• ja biram = I choose, but with extra emphasis, contrast, or stress

Using ja is possible, but not necessary here.

It can mean either, depending on context.

Croatian present tense often covers both:

• I choose
• I am choosing

So biram drugi paket can mean:

• a general present decision
• a choice happening now

Also, biram comes from birati, which is an imperfective verb. That makes it suitable for an ongoing process, repeated action, or general present situation.

It actually is in the correct object case: accusative. It just happens to look the same as the nominative.

In Croatian, for many masculine inanimate singular nouns:

• nominative = accusative

So:

• paket = nominative
• paket = accusative

The same is true for drugi here:

• drugi paket can be nominative
• drugi paket can also be accusative

You can compare that with an animate masculine noun, where the accusative usually changes:

• vidim novog čovjeka

It can mean either, depending on context.

In this sentence, it most naturally means another package or a different package, because the speaker is rejecting one plan and choosing a different one.

If the context were about order in a list, then drugi could mean second.

In telecom language, they are related but not exactly identical.

• tarifa usually refers to the tariff/plan/rate structure
• paket usually refers to a package/bundle/plan option

In everyday use, speakers may use these somewhat loosely, especially when talking about mobile services. In this sentence, the idea is that the current mobile tariff/plan is too expensive, so the speaker is choosing another package/plan.

Because stoga connects the second clause to the first with a logical meaning like therefore.

The comma helps separate:

• the reason: Mobilna tarifa je skupa
• the result/conclusion: stoga biram drugi paket

This punctuation is natural and standard. You could also write it as two sentences:

• Mobilna tarifa je skupa. Stoga biram drugi paket.

Because Croatian has no articles like English the and a/an.

So Croatian nouns appear without articles, and the listener understands the meaning from context.

That means:

• mobilna tarifa could mean the mobile tariff or a mobile tariff
• drugi paket could mean another package or the second package, depending on context

English needs articles, but Croatian does not.