Sprzedawca energii sprawdza grupę taryfową.

Because energii is the genitive singular form of energia.

In Polish, when one noun describes another noun in the sense of of something, Polish often uses the genitive:

• sprzedawca energii = seller of energy
• literally, seller + of energy

So:

• energia = energy
• energii = of energy

This is very common in Polish noun phrases:

• producent żywności = producer of food
• sprzedawca samochodów = seller of cars

Because this phrase is the direct object of the verb sprawdza.

The dictionary form is:

• grupa taryfowa = tariff group

But after sprawdza in this sentence, it has to be in the accusative case:

• grupa taryfowa → grupę taryfową

This happens because sprawdzać takes a direct object:

• sprawdzać coś = to check something

So:

• sprawdza grupę taryfową = checks the tariff group

Sprawdza is:

• 3rd person singular
• present tense
• imperfective

It comes from the verb sprawdzać = to check

So sprawdza means:

• he checks
• she checks
• it checks

Because Polish usually drops subject pronouns, the noun sprzedawca tells us who is doing the action.

Yes. In Polish, the present tense often covers both meanings.

So sprawdza can mean:

• checks
• is checking

The exact meaning depends on context.

For example:

• Codziennie sprawdza grupę taryfową. = He checks the tariff group every day.
• Teraz sprawdza grupę taryfową. = He is checking the tariff group now.

The subject is sprzedawca energii.

More exactly:

• sprzedawca is the core noun: seller
• energii describes it: of energy

Together, they mean energy seller or energy supplier

The object is grupę taryfową:

• this is the thing being checked

So the structure is:

• Sprzedawca energii = subject
• sprawdza = verb
• grupę taryfową = object

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

So:

• sprzedawca can mean a seller or the seller
• grupę taryfową can mean a tariff group or the tariff group

Context tells you which meaning is intended.

That is completely normal in Polish.

No, Polish word order is more flexible than English because case endings show grammatical roles.

The neutral, most straightforward order here is:

• Sprzedawca energii sprawdza grupę taryfową.

But other orders are possible, for example for emphasis:

• Grupę taryfową sprawdza sprzedawca energii.
• Sprzedawca energii grupę taryfową sprawdza.

These alternatives are grammatically possible, but they sound more marked or stylistically different. For a learner, the original subject–verb–object order is the safest choice.

Yes, it is natural Polish.

Literally, it means seller of energy. In real-world English, depending on context, you might also translate it as:

• energy seller
• energy supplier
• electricity supplier if the context is specifically electricity

So the Polish is natural, but the best English translation can vary depending on the situation.

In Polish, adjectives often come before nouns, but in formal, technical, administrative, or set expressions they can also come after the noun.

Grupa taryfowa is a standard technical-style phrase meaning tariff group.

So this order sounds natural:

• grupa taryfowa

If you changed the order to taryfowa grupa, it would sound less standard in this context and more stylistically marked.

A rough English-friendly guide is:

Sprzedawca energii sprawdza grupę taryfową
sp-sheh-DAF-tsa e-NER-gyee SPFRAHV-ja GROO-peh ta-ri-FO-vow

A few useful sound notes:

• rz sounds roughly like the s in measure, but harder
• w sounds like English v
• c sounds like ts
• ę is a nasal vowel; at the end of grupę, many learners can approximate it as eh
• ą in taryfową is nasal; learners often approximate it as ow at first

Stress in Polish is usually on the second-to-last syllable:

• sprzeDAWca
• eNERgii
• sprawDZA
• taRYfową

Here are the dictionary forms:

• sprzedawca → dictionary form: sprzedawca
  meaning: seller

• energii → dictionary form: energia
  meaning: energy

• sprawdza → dictionary form: sprawdzać
  meaning: to check

• grupę → dictionary form: grupa
  meaning: group

• taryfową → dictionary form: taryfowy / taryfowa / taryfowe
  meaning: tariff-related, tariff

This is a useful habit in Polish: when you see a sentence, try to identify both the form in the sentence and the dictionary form.