U pokladny nemám drobné, proto platím kartou.

It means at the checkout, at the cash desk, or by the cashier.

The preposition u usually means by, near, or at, and pokladna is the cash register / checkout / cashier’s desk. So u pokladny sets the scene: the speaker is standing at the place where payment happens.

Because the preposition u requires the genitive case.

The basic form is pokladna. After u, it changes to genitive singular:

• pokladna = checkout, cash desk
• u pokladny = at the checkout

This is a very common Czech pattern:

• u stolu = at the table
• u domu = by the house
• u pokladny = at the checkout

Here drobné means small change, especially coins or small money.

It is very commonly used on its own, but it is really a shortened form of drobné peníze = small money / change.

So:

• Mám drobné. = I have change.
• Nemám drobné. = I don’t have any change.

In everyday Czech, drobné is much more natural than saying the full drobné peníze every time.

Because Czech often does not need a separate word for any when the verb is negative.

So:

• Nemám drobné. literally = I don’t have change
• natural English translation = I don’t have any change

If you want, you can make it more explicit with žádné:

• Nemám žádné drobné. = I don’t have any change at all.

But in normal speech, Nemám drobné is already completely natural.

This is a very important distinction:

• proto = therefore / so / for that reason
• protože = because

In your sentence:

• U pokladny nemám drobné, proto platím kartou.
• I don’t have change at the checkout, therefore I’m paying by card.

So proto introduces the result.

Compare:

• Platím kartou, protože nemám drobné.
• I’m paying by card because I don’t have change.

Same basic idea, but the sentence is structured differently.

Because the sentence contains two clauses:

• U pokladny nemám drobné
• proto platím kartou

In Czech, when clauses like this are joined and the second clause expresses a consequence, a comma is normally used.

So the comma here is standard and correct.

Because Czech uses the instrumental case to show the means or method of doing something.

The basic noun is:

• karta = card

The instrumental singular is:

• kartou = by card / with a card

So:

• platím kartou = I pay by card / I’m paying with a card

This is a very common Czech pattern:

• jedu vlakem = I go by train
• píšu perem = I write with a pen
• platím kartou = I pay by card

It can mean both, depending on context.

Czech present tense often covers both:

• I pay
• I am paying

Here, because the situation is happening at the checkout right now, platím kartou is best understood as:

• I’m paying by card

But in another context, platím kartou could also mean a habitual action:

• Usually, I pay by card.

The context tells you which meaning is intended.

Yes. Czech word order is more flexible than English word order, but different orders create different emphasis.

The given sentence is very natural:

• U pokladny nemám drobné, proto platím kartou.

It starts with u pokladny to set the situation first.

Other versions are possible, for example:

• Nemám u pokladny drobné, proto platím kartou.
• Proto platím kartou, protože u pokladny nemám drobné.

These are understandable, but the original version sounds smooth and natural for everyday use.

So yes, word order can change, but it is not random: it changes what is being highlighted.

Not exactly, though they can overlap in some situations.

• u pokladny = at/by the checkout
• na pokladně can sometimes mean at the checkout, but it can also suggest on the cash register / in the register system / at the cashier station

In a shopping situation, u pokladny is the most natural way to say at the checkout when you mean the physical place where you are standing.

So for this sentence, u pokladny is the best choice.

Yes, it sounds natural and idiomatic.

A Czech speaker would easily say:

• U pokladny nemám drobné, proto platím kartou.

In very casual speech, someone might also say:

• Nemám drobné, tak platím kartou.

That version is a bit more conversational because tak or takže often sounds less formal than proto.

So the original sentence is correct and natural, just slightly more neat or written in tone than the most casual spoken version.