Repertoár je na internetu.

Je is the 3rd person singular form of the verb být (to be).

So:

• repertoár = repertoire / program
• je = is
• na internetu = on the internet

Because repertoár is singular, Czech uses je = is.

Compare:

• Repertoár je na internetu. = The repertoire is on the internet.
• Repertoáry jsou na internetu. = The repertoires are on the internet.

After na when it means in/on a place, Czech uses the locative case.

So:

• base form: internet
• locative after na: na internetu

That is why the word changes.

This is a very common pattern:

• na stole = on the table
• na náměstí = in the square / on the square
• na internetu = on the internet

So internetu is the locative singular form of internet.

In Czech, the usual idiomatic expression is na internetu = on the internet.

Even though English speakers might wonder whether it should be something like in the internet, Czech normally says na internetu.

This is just the standard collocation:

• být na internetu = to be on the internet
• najít něco na internetu = to find something on the internet

So this is something best learned as a fixed expression.

Repertoár is masculine inanimate.

You can often tell from the ending and from how it behaves grammatically. Since it is masculine inanimate singular, it works naturally with:

• ten repertoár = that repertoire
• repertoár je = the repertoire is

This matters later when you learn adjectives and other case forms.

Czech has no articles, so there is no separate word for a/an or the.

So Repertoár je na internetu. could mean:

• The repertoire is on the internet
• A repertoire is on the internet

In real use, context usually makes it clear. In this sentence, English would most naturally say The repertoire is on the internet, but Czech does not need a word for the.

Yes. Czech word order is flexible, although different orders can change the emphasis.

The neutral version is:

• Repertoár je na internetu.

You could also say:

• Na internetu je repertoár.

That still means basically the same thing, but it may place more focus on where it is.

Very roughly:

• Repertoár je na internetu. = neutral statement about the repertoire
• Na internetu je repertoár. = emphasis on the location

A simple learner-friendly pronunciation guide is:

RE-per-to-aar ye na IN-ter-ne-tu

A few useful points:

• Czech stress is usually on the first syllable of each word:
  • REpertoár
  • INternetu
• á is a long vowel, so repertoár ends with a longer a sound.
• j in Czech is pronounced like English y in yes, so je sounds roughly like ye.

Usually, no. In modern Czech, internet is commonly written with a lowercase i:

• na internetu

That is the normal spelling in everyday usage.

Yes, you may hear or see online, and it can work in many contexts. But Repertoár je na internetu is a very natural Czech sentence.

The two are similar, but not always identical in tone:

• je na internetu = is on the internet / available on the internet
• je online = is online

So na internetu is a very standard and safe choice here.

It is neutral. It is not especially formal or informal.

You could use it in:

• everyday speech
• a website
• a notice
• general written communication

So it is a good basic model sentence to learn.