Moja ciocia była kiedyś sekretarką, a dziś jest recepcjonistką w hotelu.

Because ciocia is a feminine noun, and the possessive word my has to agree with it in gender, number, and case.

So:

• mój = my, for a masculine singular noun
• moja = my, for a feminine singular noun
• moje = my, for a neuter singular noun

Examples:

• mój wujek = my uncle
• moja ciocia = my aunt
• moje dziecko = my child

So moja ciocia is correct because ciocia is feminine.

Yes. Ciocia means aunt, but it often feels a bit warmer or more familiar, a little like auntie in English.

In everyday Polish, people very commonly say ciocia.
A more formal or less affectionate word is teta, but that is much less common in normal conversation.

So in this sentence, moja ciocia is a very natural way to say my aunt.

Była is the past tense of być (to be) for a feminine singular subject.

Since ciocia is feminine singular, the verb must match it:

• był = he/it was
• była = she/it was
• było = it was (neuter)

So:

• Moja ciocia była... = My aunt was...

If the subject were masculine, it would change:

• Mój wujek był sekretarzem. = My uncle was a secretary.

Here kiedyś means once, at one time, or more naturally in the past / back then.

So:

• Moja ciocia była kiedyś sekretarką
  = My aunt used to be a secretary / My aunt was once a secretary

It does not mean one exact time like once at 3 p.m. It is more general and refers to some period in the past.

Depending on context, kiedyś can also mean sometime in the future, but in this sentence, because of była (was), it clearly refers to the past.

Because after the verb być (to be), when you say what someone is by profession, role, or identity, Polish usually uses the instrumental case.

That is why:

• sekretarka → sekretarką
• recepcjonistka → recepcjonistką

So:

• była sekretarką = she was a secretary
• jest recepcjonistką = she is a receptionist

This is one of the most important patterns to learn in Polish:

• On jest nauczycielem. = He is a teacher.
• Ona jest lekarką. = She is a doctor.
• Oni są studentami. = They are students.

In standard Polish, jest recepcjonistką is the correct form here.

Using the nominative form after to be sometimes appears in certain contexts, especially when identifying someone very directly, but with professions and roles, Polish normally prefers the instrumental:

• Jest recepcjonistką. = She is a receptionist.
• Była sekretarką. = She was a secretary.

So for a learner, the safe rule is:

After forms of być, professions are usually in the instrumental case.

Here a links two ideas by contrast:

• Moja ciocia była kiedyś sekretarką, a dziś jest recepcjonistką w hotelu.

A natural English translation would be:

• My aunt used to be a secretary, but today she is a receptionist in a hotel.

Sometimes a can be translated as and, but very often it has a slight contrastive sense, like:

• whereas
• while
• but

In this sentence, a dziś means something like but today / whereas today.

Yes. Dziś and dzisiaj both mean today.

• dziś is shorter
• dzisiaj is slightly fuller, but both are very common and natural

So you could also say:

• Moja ciocia była kiedyś sekretarką, a dzisiaj jest recepcjonistką w hotelu.

The meaning does not really change.

Because w can mean in or at, and when it refers to a location, the following noun usually goes into the locative case.

So:

• hotel → w hotelu = in a hotel / at a hotel

Compare:

• w domu = at home / in the house
• w biurze = in the office
• w szkole = at school

So recepcjonistką w hotelu means a receptionist in a hotel.

Yes. Recepcjonistka is the feminine form.

The masculine form is:

• recepcjonista

Because the aunt is female, the sentence uses the feminine noun:

• jest recepcjonistką

The same thing happens with sekretarka, which is also feminine.
A masculine equivalent would be sekretarz.

Examples:

• Ona jest recepcjonistką. = She is a receptionist.
• On jest recepcjonistą. = He is a receptionist.

Yes. Polish word order is more flexible than English word order, because case endings show the grammatical relationships.

The original sentence is very natural:

• Moja ciocia była kiedyś sekretarką, a dziś jest recepcjonistką w hotelu.

But you could also say:

• Kiedyś moja ciocia była sekretarką, a dziś jest recepcjonistką w hotelu.
• Moja ciocia była sekretarką kiedyś, a dziś jest recepcjonistką w hotelu.
  (possible, but less neutral)

The most natural version usually puts kiedyś before or near the verb, and dziś at the start of the second clause for contrast.

The letter ą is a nasal vowel. At the end of a word, in normal modern pronunciation, it is often pronounced roughly like -om or -ɔ̃, depending on the speaker and context.

So:

• sekretarką
• recepcjonistką

will often sound approximately like:

• sekretarkom
• recepcjonistkom

That does not mean they are spelled with -om. The spelling stays -ą.

For learners, the important thing is:

• recognize -ą as a normal instrumental singular ending for many feminine nouns
• do not be surprised if native pronunciation sounds a bit like -om

Because Polish often omits subject pronouns when they are not necessary.

In English, you need:

• My aunt was once a secretary, and today she is a receptionist in a hotel.

In Polish, the subject is already clear from moja ciocia, so you do not need ona (she) in the second part.

So Polish naturally says:

• Moja ciocia była kiedyś sekretarką, a dziś jest recepcjonistką w hotelu.

You could add ona for emphasis, but normally it is omitted.