Pokud jde o roaming, ptejte se mobilního operátora na příplatek.

Pokud jde o is a very common Czech phrase meaning as for, regarding, or when it comes to.

So:

• Pokud jde o roaming... = When it comes to roaming...

Even though jde comes from jít (to go), in the expression jít o něco it means to concern / to be about, not physical movement.

Yes, jít usually means to go, but Czech often uses verbs in idiomatic ways.

Here, jít o + accusative means:

• to be about
• to concern
• to involve

Examples:

• Jde o peníze. = It’s about money.
• Jde o bezpečnost. = It concerns safety.

So Pokud jde o roaming literally means something like If it goes about roaming, but natural English is As for roaming or When it comes to roaming.

Ptejte se is the imperative form of ptát se (to ask).

More specifically, it is the:

• 2nd person plural imperative: ask!
• and it is also used as the polite singular imperative

So it can mean:

• ask! when speaking to several people
• please ask when speaking politely to one person

In public instructions, notices, and formal recommendations, this is very common.

Related forms:

• ptej se = ask! (singular, informal)
• ptejte se = ask! / please ask (plural or polite)

Because the verb is ptát se, not just ptát.

In Czech, some verbs regularly appear with se or si. This is sometimes called a reflexive form, but with many verbs it is simply part of how the verb is used.

So:

• ptát se = to ask
• ptejte se = ask

You should learn it as a whole verb:

• ptát se koho na co = to ask someone about something

The se usually comes right after the imperative:

• Ptejte se...

That word order is normal.

Because ptát se requires the person you ask to be in the genitive case.

So:

• ptát se koho/čeho = to ask whom/of what

That is why:

• mobilní operátor = nominative
• mobilního operátora = genitive

In this sentence:

• ptejte se mobilního operátora = ask the mobile operator

This is an important pattern:

• ptát se učitele
• ptát se lékaře
• ptát se kamaráda

All of these use the person in the genitive.

Here na příplatek is the second part of the pattern used with ptát se:

• ptát se koho na co = to ask someone about something

So:

• mobilního operátora = the person being asked
• na příplatek = the thing you are asking about

After na in this pattern, Czech uses the accusative.

So:

• příplatek is in the accusative singular
• for this noun, the accusative looks the same as the nominative

Example of the same structure:

• Ptal jsem se učitele na test.
• I asked the teacher about the test.

Příplatek means an extra charge, surcharge, or additional fee.

It is not the basic price itself, but something added on top.

In the context of roaming, it suggests:

• an extra roaming fee
• a surcharge connected with roaming

A useful distinction:

• poplatek = fee/charge in general
• příplatek = extra charge, additional payment

So na příplatek means about the surcharge / about any extra fee.

Not here.

That is a very good question, because in Czech the meaning changes depending on the preposition.

• na příplatek here means about the surcharge
• za příplatek usually means for an extra fee

Compare:

• Ptejte se na příplatek. = Ask about the surcharge.
• Dostanete to za příplatek. = You can get it for an extra fee.

So in your sentence, na is part of the verb pattern ptát se někoho na něco.

In Pokud jde o roaming, roaming is in the accusative, because jít o něco takes the accusative.

The noun roaming is a masculine inanimate loanword. In the singular, its accusative is the same as its nominative, so the form stays roaming.

That is why you do not see a different ending here.

So:

• roaming = nominative
• roaming = accusative as well

Because Pokud jde o roaming is an introductory phrase at the start of the sentence.

Czech often separates this kind of introductory expression with a comma, especially when it sets the topic:

• Pokud jde o cenu, ...
• Pokud jde o kvalitu, ...
• Pokud jde o roaming, ...

So the comma helps mark:

1. the topic: as for roaming
2. the main instruction: ask the mobile operator about the surcharge

Both are possible in many contexts, but they are not exactly the same.

• ptát se / ptejte se = imperfective
• zeptat se / zeptejte se = perfective

Very roughly:

• ptejte se can sound like ask / feel free to ask / ask as needed
• zeptejte se can sound more like ask once / make the inquiry

In notices, advice, and general instructions, ptejte se is very natural because it has a broader, more general sense.

So this sentence sounds like a standing recommendation:

• If it’s about roaming, ask your mobile operator about the surcharge.

Not just one single completed question, but general advice.

Yes. It is either:

• plural (you all ask), or
• polite singular (please ask to one person)

Czech uses plural forms to address one person politely, much like some other European languages.

So in public-facing language, service language, instructions, and customer advice, ptejte se sounds appropriate and neutral.

If you were speaking informally to one friend, you would say:

• Ptej se mobilního operátora na příplatek.

But that would not fit the style of a public notice or general recommendation as well.