Používám mobilní data.

In Czech, the subject pronoun is often omitted because the verb ending already shows who is doing the action.

• Používám = I use / I am using
• The ending -ám tells you it is 1st person singular: I

So Já používám mobilní data is possible, but já is usually only added for emphasis, contrast, or clarity.

Používám is the 1st person singular present tense of the verb používat.

So:

• používat = to use
• používám = I use / I am using

Czech does not have a separate verb form for I use versus I am using, so the same form can cover both meanings depending on context.

Czech does not have a special continuous tense like English am using. The present tense often covers both:

• a general habit: I use mobile data
• an action happening now: I’m using mobile data

Whether it means a habitual action or something happening right now depends on context.

The dictionary form is používat, which is imperfective.

That matters because Czech verbs often come in aspect pairs:

• používat = imperfective, ongoing/repeated/general use
• použít = perfective, use once / apply / make use of

In this sentence, používám sounds natural because it describes an ongoing or general action.

For example:

• Používám mobilní data. = I use / I’m using mobile data.
• Použiju mobilní data. = I’ll use mobile data.
  The perfective present often has a future meaning.

It is in the accusative case because it is the direct object of používám.

The verb používat takes the accusative:

• používat něco = to use something

So:

• mobilní data = the thing being used

In this sentence, the accusative form looks the same as the nominative form.

Because data is a plural neuter noun, and in this sentence it is in the accusative plural. For many neuter plural nouns in Czech, the nominative plural and accusative plural are the same.

So:

• nominative plural: data
• accusative plural: data

That is why the word does not change here.

A useful note for English speakers: in Czech, data is normally treated as a plural noun, not as an uncountable mass noun the way English often uses data.

For example:

• Data jsou důležitá. = Data are important.

Because adjectives in Czech must agree with the noun they describe in:

• gender
• number
• case

Here, data is neuter plural, and the phrase is in the accusative plural, so the adjective must match that.

Thus:

• mobilní data

In this form, mobilní is the correct agreeing adjective form.

Here it means mobile data / cellular data.

So mobilní data is the normal way to talk about using your phone network’s data connection.

It does not usually mean files or information stored on a mobile phone. For that, Czech would use a different expression depending on the context.

Yes, Používám mobilní data. is natural and correct.

It can be used in contexts like:

• you do not have Wi‑Fi
• you are telling someone you are online through your phone network
• you generally rely on mobile data

Depending on context, other natural Czech phrases also exist, for example:

• Jsem na mobilních datech. = I’m on mobile data.
• Mám zapnutá mobilní data. = I have mobile data turned on.

But your sentence itself is perfectly fine.

Yes. Czech word order is more flexible than English, although not completely free.

The neutral order here is:

• Používám mobilní data.

But you could also hear:

• Mobilní data používám.

That version gives more emphasis or contrast to mobilní data.

For example, it might imply:

• Mobile data is what I use
  perhaps not Wi‑Fi

So the standard learner-friendly version is the original one.

A rough English-friendly guide is:

POU-zhee-vaam MO-bil-nee DA-ta

A few details:

• ou in Používám is a diphthong, roughly like oh-oo said quickly
• ž sounds like the s in measure
• í and á are long vowels
• Czech stress usually falls on the first syllable of each word

So the stress pattern is roughly:

• POUžívám
• MObilní
• DAta

No, it is not wrong. It is just less neutral.

• Používám mobilní data. = neutral, normal
• Já používám mobilní data. = I use mobile data
  with extra emphasis on I

You might add já if you are contrasting yourself with someone else:

• Ty používáš Wi‑Fi, ale já používám mobilní data.
  = You use Wi‑Fi, but I use mobile data.

Sometimes yes, but the nuance is a little different.

• mobilní data = mobile data, cellular data
• mobilní internet = mobile internet

In everyday speech, both may be understood in similar situations, but mobilní data is the more direct match for mobile data.

So if that is exactly what you want to say, Používám mobilní data. is the better choice.

The negative is:

• Nepoužívám mobilní data. = I do not use / I am not using mobile data.

Czech usually makes verbs negative by adding ne- to the beginning of the verb:

• používám → nepoužívám

That is a very common pattern in Czech.