Karta, kterou nosím v peněžence, je z nové banky.

The main clause is:

Karta ... je z nové banky.
= The card ... is from a new bank.

Inside that main clause, there is an inserted relative clause:

kterou nosím v peněžence
= which/that I carry in my wallet

So the full structure is:

subject + relative clause + predicate

• Karta = the subject
• kterou nosím v peněžence = describes karta
• je z nové banky = says something about the card

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

So:

• karta can mean a card or the card
• z nové banky can mean from a new bank or, depending on context, from the new bank

Czech usually leaves definiteness to context, word order, or emphasis.

Because který changes form according to gender, number, and case.

It refers back to karta, so it must be feminine singular. But its case depends on its role inside the relative clause:

• nosím koho/co? = I carry whom/what?
• The thing being carried is the card.
• So the relative pronoun is the direct object of nosím.

For a feminine singular direct object, the correct form is kterou.

So:

• která = nominative feminine singular
• kterou = accusative feminine singular

Normally no.

In English, you can say:

• The card that I carry in my wallet
• The card I carry in my wallet

In Czech, the relative pronoun is usually required:

• Karta, kterou nosím v peněžence...

Leaving it out would sound wrong or very unnatural in standard Czech.

Because in Czech, subordinate clauses are generally separated by commas, including relative clauses introduced by words like který.

So when the relative clause is inserted in the middle of the sentence, Czech puts a comma on both sides:

Karta, kterou nosím v peněžence, je z nové banky.

This is different from English, where commas depend more on whether the clause is restrictive or non-restrictive. Czech punctuation is stricter here.

Nosím is from nosit, which usually means to carry / to wear / to have on oneself regularly.

Here it suggests that the speaker keeps or carries the card in the wallet.

It is natural because it describes a repeated or usual situation.

Compare:

• nosím = I carry / I keep on me / I regularly have with me
• mám = I have
• nesu = I am carrying (right now, in one concrete instance)

So kterou nosím v peněžence sounds like which I carry in my wallet in a habitual sense.

Yes, you could.

There is a slight difference in feel:

• kterou nosím v peněžence = the card I carry in my wallet
• kterou mám v peněžence = the card I have in my wallet

mám focuses more on possession/location.
nosím focuses more on carrying it around as part of your everyday routine.

Both are possible, but nosím is a good natural choice here.

Because the preposition v meaning in takes the locative case when it expresses location.

The dictionary form is:

• peněženka = wallet

In the locative singular, it becomes:

• v peněžence = in the wallet

So this is simply the correct case after v for location.

Because the preposition z means from / out of and takes the genitive case.

So:

• banka = bank
• z banky = from the bank
• z nové banky = from a new bank

This phrase expresses origin or source.

Because the adjective must agree with the noun it describes in gender, number, and case.

The noun is:

• banka = feminine singular

But after z, it must be in the genitive singular:

• banky

So the adjective also has to be feminine singular genitive:

• nové banky

That is why it is nové, not nová.

Yes, Czech word order is fairly flexible, but this version is neutral and natural.

Karta, kterou nosím v peněžence, je z nové banky.

This puts the topic first: the card.

You can change word order for emphasis, but then the sentence may sound more marked or literary. For everyday neutral Czech, the given order is very good.

In colloquial speech, some speakers do use co in relative clauses:

• Karta, co nosím v peněžence...

But in standard written Czech, kterou is the correct and better choice.

So for a learner, especially in writing, use:

• kterou

not

• co

unless you specifically want a colloquial spoken style.