Koristim karticu svakog dana.

Koristim is the 1st person singular present tense of koristiti, so it means I use.

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

In this sentence, the subject ja is not stated because Serbian often leaves subject pronouns out when the verb already makes the person clear.

It can mean either, depending on context.

In this sentence, because of svakog dana (every day), the natural reading is I use the card every day, meaning a habitual action.

Serbian does not usually make the same strict distinction that English does between:

• I use
• I am using

The present tense often covers both, and context tells you which one is meant.

Because karticu is the accusative singular form, and the card is the direct object of the verb koristim.

The base form is:

• kartica = card

But after a verb like use, Serbian puts the direct object in the accusative:

• koristim karticu = I use the card

This is a very common pattern. For many feminine nouns ending in -a, the accusative singular ends in -u:

• knjiga → knjigu
• torba → torbu
• kartica → karticu

Kartica usually means card, especially a small practical card such as:

• a bank card
• a credit/debit card
• an ID-type card
• a membership card

In everyday Serbian, kartica is the normal word for this kind of card.

Both svakog dana and svaki dan can mean every day.

In your sentence, svakog dana uses the genitive form:

• svakog = genitive of svaki
• dana = genitive of dan

This is a very common Serbian time expression.

So you may hear both:

• Koristim karticu svakog dana.
• Koristim karticu svaki dan.

Both are correct and natural. Svakog dana can sound a little more formal or stylistically fuller, but in many situations the difference is small.

Yes, the basic form is dan (day).

Here, though, it appears as dana because the phrase svakog dana uses the genitive singular.

So:

• dan = day
• svakog dana = every day

This is one of those time expressions that learners simply get used to as a set phrase.

No, Serbian word order is fairly flexible.

The sentence:

• Koristim karticu svakog dana.

is perfectly natural and neutral.

But you could also say:

• Svakog dana koristim karticu.

This puts more focus on every day.

Because Serbian has case endings, word order can move around more freely than in English, although not every order sounds equally natural in every context.

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

So karticu can mean:

• a card
• the card

Context tells you which one is meant.

That is why one Serbian sentence can correspond to more than one English translation.

Yes. Koristiti is the standard, everyday verb for to use.

It is the most natural choice in a sentence like this.

There are other related verbs, but they can sound more formal or have slightly different meanings. For example:

• upotrebiti = to use, to employ
• iskoristiti = to make use of, to take advantage of, sometimes to use up

For a simple everyday statement like I use the card every day, koristiti is the best choice.

Yes. Koristiti is an imperfective verb, which makes sense here because the action is:

• repeated
• habitual
• ongoing as a pattern

Since the sentence says every day, Serbian naturally uses the imperfective verb.

A perfective verb would not fit as well for a general repeated habit like this.

Roughly:

• kar-ti-tsu

A useful detail is that Serbian c is pronounced like ts in cats.

So:

• kartica ≈ KAR-ti-tsa
• karticu ≈ KAR-ti-tsu

That ts sound is important, because English speakers often want to pronounce c like k or s, but here it is ts.

Yes, you could say:

• Ja koristim karticu svakog dana.

But normally ja is unnecessary, because koristim already tells you that the subject is I.

Adding ja usually gives extra emphasis, such as:

• I use the card every day.
• As for me, I use the card every day.

So the version without ja is the more neutral one.