Jednym z moich obowiązków jest kontakt z klientem i szybkie wysyłanie faktur.

It literally means one of my duties.

• jednym = one in the instrumental singular
• z = of / from among
• moich obowiązków = my duties in the genitive plural

This is a very common Polish pattern:

jeden / jedna / jedno z + genitive plural
= one of ...

So:

• Jednym z moich obowiązków jest...
  = One of my duties is...

Even though jednym is instrumental, the noun after z is in the genitive plural because of the fixed pattern jeden z czegoś = one of something.

Because in this structure, jeden changes form.

The base form is:

• jeden = one

But here it becomes:

• jednym

That is the instrumental singular form, used in the expression Jednym z... jest...

This may feel strange at first, but the whole phrase behaves like:

• Jednym z problemów jest... = One of the problems is...
• Jednym z powodów jest... = One of the reasons is...

So jednym is just the correct grammatical form in this sentence pattern.

Because after z in the meaning of / from among, Polish uses the genitive.

So:

• moje obowiązki = my duties nominative
• moich obowiązków = of my duties genitive

In the phrase jednym z moich obowiązków, the meaning is literally:

• one from among my duties

That is why obowiązki changes to obowiązków, and moje changes to moich.

Because the subject is one of my duties, and that idea is grammatically singular.

So even though duties is plural inside the phrase, the real subject is:

• Jednym ... jest ...
  = One ... is ...

That is why Polish uses:

• jest = is

not

• są = are

Compare:

• Jednym z moich obowiązków jest kontakt z klientem.
  = One of my duties is contact with the client/customer.

But:

• Moje obowiązki to kontakt z klientem i wysyłanie faktur.
  = My duties are contact with the client/customer and sending invoices.

In that second sentence, the subject is plural, so the structure changes.

Because they mean different things and require different cases.

1. z in Jednym z moich obowiązków

Here z means of / from among.
It takes the genitive:

• z moich obowiązków

2. z in kontakt z klientem

Here z means with.
It takes the instrumental:

• z klientem

So although both are spelled z, they are functioning differently.

This is very common in Polish:

• jeden z uczniów = one of the students
• rozmawiać z uczniem = to talk with a student

Same preposition, different meaning, different case.

Because kontakt z kimś uses the instrumental case after z meaning with.

So:

• klient = nominative
• klienta = genitive/accusative
• klientem = instrumental

And the phrase is:

• kontakt z klientem = contact with a client / customer contact

Other examples:

• rozmowa z klientem = conversation with a client
• spotkanie z klientem = meeting with a client
• współpraca z klientem = cooperation with a client

Usually it means customer contact in general, not necessarily one specific client.

In business Polish, singular nouns are often used in a general, abstract way:

• kontakt z klientem = customer contact
• obsługa klienta = customer service
• rozmowa z klientem = talking to customers / client communication, depending on context

So in this sentence, it most likely means that one of the speaker’s duties is dealing with customers or being in contact with clients generally.

If you wanted to make the plural explicit, you could say:

• kontakt z klientami = contact with clients

But the singular version is very natural and common.

Because here Polish uses a verbal noun, not an infinitive verb.

• wysyłać = to send
• wysyłanie = sending

So:

• szybkie wysyłanie faktur
  literally = quick sending of invoices

This matches the earlier noun phrase:

• kontakt z klientem
• szybkie wysyłanie faktur

Both are noun-like expressions listing duties.

If you said szybko wysyłać faktury, that would mean to send invoices quickly, but it would not fit as neatly into this noun-based list of responsibilities.

Polish often uses verbal nouns in formal or professional language:

• prowadzenie dokumentacji = keeping documentation
• odbieranie telefonów = answering phones
• przygotowywanie raportów = preparing reports

Because wysyłanie is a noun, and adjectives modify nouns.

• wysyłanie = sending, a noun
• szybkie wysyłanie = quick sending

If it were a verb, then you would use an adverb:

• szybko wysyłam faktury = I send invoices quickly

But in the original sentence, wysyłanie is not a verb form meaning I send. It is a noun meaning sending, so it needs the adjective:

• szybkie

This is exactly like English:

• quick sending of invoices
  not
• quickly sending if you want a noun phrase parallel to contact with the client

Because wysyłanie is a verbal noun, and when a verbal noun comes from a transitive verb, its object is usually put in the genitive.

Compare:

With a normal verb

• wysyłać faktury = to send invoices

Here faktury is the direct object.

With a verbal noun

• wysyłanie faktur = sending of invoices

Here the object becomes genitive:

• faktur

This is a very common pattern in Polish:

• czytać książkę = to read a book

• czytanie książki = reading a book

• pisać raporty = to write reports
• pisanie raportów = writing reports

So wysyłanie faktur is completely regular.

Yes, it sounds fairly neutral to formal, especially because of wysyłanie faktur.

This style is very common in:

• job descriptions
• CVs
• cover letters
• workplace conversations
• descriptions of responsibilities

A more conversational version might be:

• Jednym z moich obowiązków jest kontakt z klientem i szybkie wysyłanie faktur.

This sentence is already natural, but in casual speech people might also say things like:

• Zajmuję się kontaktem z klientem i szybkim wysyłaniem faktur.
• Do moich obowiązków należy kontakt z klientem i szybkie wysyłanie faktur.

All of these are natural, but the original sounds especially suitable for professional contexts.

Yes, absolutely.

A very common alternative is:

• Do moich obowiązków należy kontakt z klientem i szybkie wysyłanie faktur.

This means:

• My duties include contact with the client and quick sending of invoices.
• or more naturally, My responsibilities include customer contact and the prompt sending of invoices.

The difference is mainly stylistic:

• Jednym z moich obowiązków jest...
  = One of my duties is...
• Do moich obowiązków należy...
  = Among my duties is / My duties include...

Both are correct and common.

Yes, Polish word order is flexible, though some orders sound more natural than others.

The original sentence:

• Jednym z moich obowiązków jest kontakt z klientem i szybkie wysyłanie faktur.

is very natural and neutral.

You could also say:

• Kontakt z klientem i szybkie wysyłanie faktur to jeden z moich obowiązków.

This puts more emphasis on the duties themselves first.

Both are correct. The difference is mostly about emphasis and style, not basic meaning.

A natural translation would be:

• One of my duties is customer contact and the prompt sending of invoices.

Other possible translations:

• One of my responsibilities is contacting clients and sending invoices quickly.
• One of my duties is dealing with customers and sending invoices promptly.

The exact best version depends on context:

• kontakt z klientem can be contact with the client, customer contact, or dealing with customers
• szybkie wysyłanie faktur can be quick sending of invoices, sending invoices promptly, or prompt invoice dispatch

So the sentence is quite flexible in translation, even though the Polish structure is clear.