Lubię gramatykę, ale moja siostra woli słownictwo.

Lubić is the infinitive, meaning to like.
In the sentence, the speaker is saying I like, so Polish uses the 1st person singular present-tense form:

• lubić = to like
• lubię = I like

Polish verbs change their endings depending on the person, similar to how English changes to be into am / is / are.

Because it is the direct object of the verb lubię, so it has to be in the accusative case.

The basic form is:

• gramatyka = grammar

But after lubić, a direct object is needed, so the noun changes:

• lubię gramatykę = I like grammar

This is a very common pattern for feminine nouns ending in -a:

• nominative: -a
• accusative: -ę

For example:

• książka → książkę
• kawa → kawę
• gramatyka → gramatykę

Because słownictwo is a neuter noun, and in the singular, many neuter nouns have the same form in the nominative and accusative.

So:

• nominative: słownictwo
• accusative: słownictwo

That is why you get:

• moja siostra woli słownictwo

Even though it is also a direct object, its form does not change.

They come from different verbs:

• lubię comes from lubić = to like
• woli comes from woleć = to prefer

So the sentence is not repeating the same idea exactly. It says:

• I like grammar
• but my sister prefers vocabulary

Also, woli is the 3rd person singular form:

• ja wolę = I prefer
• on / ona woli = he / she prefers

Notice that wolę and woli are irregular-looking compared with the infinitive woleć, so they are worth memorizing.

Because moja has to agree with siostra in gender, number, and case.

• siostra is feminine singular
• so the possessive adjective must also be feminine singular
• therefore: moja siostra = my sister

Compare:

• mój brat = my brother
• moja siostra = my sister
• moje dziecko = my child

So mój / moja / moje all mean my, but they change to match the noun.

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

So Polish simply says:

• Lubię gramatykę
• moja siostra woli słownictwo

Whether you translate that as grammar, the grammar, vocabulary, or the vocabulary depends on context. In this sentence, the general meaning is probably just grammar and vocabulary in a broad sense.

Polish often omits subject pronouns because the verb ending already tells you who the subject is.

• lubię already means I like
• so ja is not necessary

You could say:

• Ja lubię gramatykę

but that usually adds emphasis, something like I like grammar.

In the sentence you gave, the contrast is carried mainly by ale and by mentioning moja siostra, so ja is not needed.

Ale means but.

It connects two contrasting ideas:

• Lubię gramatykę
• ale moja siostra woli słownictwo

In Polish, ale is normally preceded by a comma when it joins two clauses, just like English often uses a comma before but.

So the punctuation is completely normal here.

Polish word order is more flexible than English word order, because case endings help show what each word is doing.

The sentence you have is the most neutral and natural version:

• Lubię gramatykę, ale moja siostra woli słownictwo.

But other orders are possible for emphasis, for example:

• Gramatykę lubię, ale moja siostra woli słownictwo.
  This puts more emphasis on grammar.

• Moja siostra woli słownictwo, ale ja lubię gramatykę.
  This changes the focus of the contrast.

So yes, the order can change, but the original sentence is a very standard one.

The letter ę is a nasal vowel, but its pronunciation can vary depending on position.

A simple learner-friendly guide:

• lubię sounds roughly like loo-BYEH with a nasal ending
• gramatykę sounds roughly like gra-ma-TIH-keh with a nasal ę at the end

A few useful points:

• ó and u sound the same in modern Polish
• y is not exactly like English ee; it is a harder vowel
• final ę is often pronounced less strongly nasal in everyday speech than learners expect

You do not need to make it sound exactly like an English vowel. It is better to aim for a clear Polish-style ending than to overdo the nasal sound.

In this sentence, they are being used as general subject areas, not as individual countable items.

So:

• gramatyka = grammar, as a field or aspect of language
• słownictwo = vocabulary, as a body of words

That is why the sentence sounds natural as a general preference statement, like saying:

• I like grammar, but my sister prefers vocabulary.

You are not talking about a grammar or some vocab words separately, but about the two areas of language learning in general.

Yes, grammatically you could, but the meaning would be different.

• Moja siostra lubi słownictwo = My sister likes vocabulary
• Moja siostra woli słownictwo = My sister prefers vocabulary

Woli creates a contrast more strongly. It suggests a comparison, especially against something else, such as grammar.

So in your original sentence, woli fits very well because the sentence contrasts two preferences:

• I like grammar
• but my sister prefers vocabulary

That sounds more natural than simply saying she likes it too.