Ona kupiła sobie nową sukienkę.

Because ona adds emphasis, contrast, or clarity.

In Polish, the verb form kupiła already tells you the subject is she, so the sentence could simply be:

Kupiła sobie nową sukienkę.

Adding ona is not wrong at all, but it can suggest something like:

• she bought herself a new dress
• as for her, she bought herself a new dress
• a contrast with someone else

So ona is often there for emphasis rather than because grammar requires it.

Kupiła is the past tense feminine singular form of the verb kupić (to buy).

Polish past tense agrees with the subject in gender and number. So:

• kupił = he bought
• kupiła = she bought
• kupiło = it bought
• kupili / kupiły = they bought

Since the subject is ona (she), the verb must be kupiła.

Because kupić is the perfective verb, and it presents the action as completed.

In this sentence, the idea is that she did buy / bought the dress as a finished event. That is why kupiła fits naturally.

Compare:

• kupiła ← from kupić: she bought it, completed action
• kupowała ← from kupować: she was buying / used to buy / bought repeatedly

So if the focus is on one completed purchase, kupiła is the normal choice.

Sobie is the dative form of the reflexive pronoun, and here it means something like for herself.

So kupiła sobie nową sukienkę means she bought a new dress for her own benefit / for herself.

Without sobie, the sentence would still be grammatical:

Ona kupiła nową sukienkę.

That simply states that she bought a new dress. Adding sobie adds the idea that it was for herself, not for someone else.

You can leave it out.

• kupiła nową sukienkę = she bought a new dress
• kupiła sobie nową sukienkę = she bought herself a new dress / bought a new dress for herself

So sobie is optional in terms of grammar, but it changes the nuance.

Because sobie and się are different cases of the reflexive pronoun.

Here, the sentence needs the meaning for herself, which uses the dative case:

• sobie = to/for oneself

Się is used in other constructions, often as a direct reflexive or in many fixed verb patterns, but not for this for herself meaning.

So:

• kupiła sobie sukienkę = she bought herself a dress

is correct, but kupiła się sukienkę is not.

Because it is the direct object of the verb, so it goes into the accusative case.

The basic dictionary form is:

• nowa sukienka = a new dress

But after kupić (to buy), the thing being bought is the direct object, so Polish uses the accusative:

• nową sukienkę

Both the adjective and the noun must change because adjectives agree with nouns in gender, number, and case.

So:

• nowa → nową
• sukienka → sukienkę

You can tell from the ending and from the role it plays in the sentence.

The noun sukienka is a feminine noun ending in -a in the nominative singular. For many feminine nouns like this, the accusative singular changes -a to -ę:

• sukienka → sukienkę

Since she bought the dress, that noun is the direct object, so accusative is exactly what you would expect.

Yes, the order can change, but adjective + noun is the most neutral and common order.

So:

nową sukienkę

is the normal way to say a new dress.

Polish word order is flexible, but changing it often changes emphasis or style. For a learner, nową sukienkę is the safest default.

No, Polish word order is fairly flexible, but different orders sound more natural in different contexts.

This sentence is a very natural one:

Ona kupiła sobie nową sukienkę.

You could also hear:

• Kupiła sobie nową sukienkę.
• Nową sukienkę sobie kupiła.
• Sobie kupiła nową sukienkę.

These alternatives are not identical in emphasis. The version you were given is neutral and easy to understand.

Yes, but it is not always as natural in this exact structure.

• kupiła sobie nową sukienkę is the most natural everyday way to say she bought herself a new dress.
• kupiła nową sukienkę dla siebie is also possible, but it sounds more explicit, as if you are stressing that it was for herself and not for someone else.

So sobie is usually the more idiomatic choice here.

Yes. In fact, that is very common in Polish.

Kupiła sobie nową sukienkę.

This is a complete sentence, because kupiła already tells us the subject is she. Polish often leaves out subject pronouns unless they are needed for emphasis, contrast, or clarity.

In modern everyday Polish, sukienka is the normal word for dress.

Learners sometimes wonder whether the -ka ending makes it sound like a diminutive. Historically, it is related to that kind of formation, but in present-day Polish sukienka is simply the ordinary word for dress, not necessarily a little dress or a cute one.