SIM kartica je spremna.

Because spreman changes to match the noun it describes.

• spreman = masculine singular
• spremna = feminine singular
• spremno = neuter singular

The noun kartica is feminine, so the adjective must also be feminine:

• SIM kartica je spremna.

This kind of agreement is very important in Croatian.

No. It is feminine because of kartica.

SIM is just an abbreviation modifying the noun. The head noun is kartica, and kartica is a feminine noun, so the whole phrase behaves as feminine.

That is why you get:

• ta SIM kartica
• nova SIM kartica
• SIM kartica je spremna

all in feminine forms.

Je is the 3rd person singular form of the verb biti (to be).

So in this sentence, it is the equivalent of is:

• je = is

Croatian often uses forms of biti just like English uses am / is / are.

Usually, in standard Croatian, no.

You normally say:

• SIM kartica je spremna.

Leaving out je would sound incomplete in ordinary neutral speech.

Croatian sometimes drops forms of to be in special styles such as headlines, notes, very informal messaging, or dramatic speech, but for a learner, the safe standard form is to keep je.

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

So:

• SIM kartica je spremna.

can mean:

• The SIM card is ready
• A SIM card is ready

Which one is meant depends on context.

This is very normal in Croatian, and learners need to get used to meaning being understood without articles.

A simple English-friendly guide:

• kartica ≈ KAR-ti-tsa
• spremna ≈ SPREM-na

A few helpful points:

• c in Croatian is pronounced like ts
• r is rolled or tapped more than in English
• stress can vary by region and standard accent patterns, but for a beginner, saying KAR-ti-tsa and SPREM-na is a good start

SIM is usually pronounced by saying the letters, like sim in many practical contexts, especially in everyday speech about phones.

Because spremna is being used as a predicate adjective, not as a regular adjective directly before the noun.

Compare:

• spremna SIM kartica = a ready SIM card
• SIM kartica je spremna = the SIM card is ready

In your sentence, the structure is:

• SIM kartica = subject
• je = verb
• spremna = predicate adjective

This is the normal pattern.

Yes, Croatian word order is more flexible than English.

• SIM kartica je spremna. = neutral, ordinary order
• Spremna je SIM kartica. = more marked, often emphasizing spremna
• SIM kartica spremna je. = possible, but less neutral and more stylistic

For everyday use, SIM kartica je spremna is the safest and most natural order.

Usually yes, but context matters.

Spreman / spremna / spremno can mean things like:

• ready
• prepared
• set
• willing, in some contexts

With a device or card, it usually means something like:

• ready for use
• prepared
• available / initialized

So in technical messages, it often means the card is recognized or usable.

Kartica means card, especially a small card like:

• bank card
• memory card
• SIM card
• ID card in some contexts

It is a diminutive-looking form historically, but in modern usage it is the normal word for many kinds of cards.

Karta usually means things like:

• ticket
• map
• playing card

So for SIM card, kartica is the correct word.

You will most commonly see:

• SIM kartica

with a space.

Sometimes you may also encounter hyphenated forms in technical or less consistent writing, but SIM kartica is the standard everyday form you should learn first.

It is in the nominative singular because it is the subject of the sentence.

The basic dictionary form is:

• kartica = nominative singular

In this sentence:

• SIM kartica = subject
• je spremna = what is being said about the subject

So the whole sentence is built around a nominative subject plus to be plus an agreeing adjective.

Yes. Both the noun and the adjective would change.

Singular:

• SIM kartica je spremna.

Plural:

• SIM kartice su spremne.

Changes:

• kartica → kartice
• je → su
• spremna → spremne

This is another example of agreement in Croatian: number matters as well as gender.