Moja siostra cierpi na ból głowy, więc dziś odpoczywa w domu.

Because moja siostra is the subject of the sentence, so both words are in the nominative case.

• siostra = sister in nominative singular
• moja = my, matching a feminine singular noun in nominative

Polish adjectives and possessive words must agree with the noun in gender, number, and case, so moja siostra is the correct subject form.

Polish does not have articles. There is no direct equivalent of English a or the.

So:

• siostra can mean a sister, the sister, or just sister
• ból głowy can mean a headache or the headache, depending on context

You understand definiteness from context rather than from a separate word.

Cierpieć na means to suffer from.

It is a common pattern in Polish:

• cierpieć na grypę = to suffer from the flu
• cierpieć na bezsenność = to suffer from insomnia
• cierpieć na ból głowy = to suffer from a headache / headaches

The verb cierpieć often sounds a bit stronger or more formal than plain English to have.

Because cierpieć commonly takes the preposition na when talking about illnesses, medical problems, or symptoms.

So the pattern is:

cierpieć na + accusative

In this sentence:

• na ból głowy

You should think of it as a fixed pattern governed by the verb, not as something you translate word-for-word from English.

Because ból is a masculine inanimate noun, and in the singular its accusative form is the same as its nominative form.

So:

• nominative: ból
• accusative: ból

Even though the case changes after na, the form happens to look the same.

Because in Polish, when one noun describes another like pain of the head, the second noun often goes into the genitive case.

So:

• głowa = head (nominative)
• głowy = of the head (genitive)

That gives:

• ból głowy = literally pain of the head

This is the normal Polish way to say headache.

It is correct, but it can sound a bit formal or suggest more serious or repeated suffering.

For everyday Polish, a very common way to say My sister has a headache would be:

• Moją siostrę boli głowa = literally My sister’s head hurts
• sometimes also Moja siostra ma ból głowy, though boli ją głowa / moją siostrę boli głowa is often more idiomatic

So this sentence is fine, but it is not the most casual, everyday phrasing.

Because Polish often drops subject pronouns when they are clear from the verb form or context.

Here, odpoczywa means he/she/it is resting. Since the sentence already mentions moja siostra, it is obvious that the person is she.

Adding ona would usually give extra emphasis:

• więc dziś ona odpoczywa w domu = so today she is resting at home

That sounds more contrastive or emphatic.

Więc means so, therefore, or thus.

It connects the two clauses:

• Moja siostra cierpi na ból głowy
• więc dziś odpoczywa w domu

The comma is normal in Polish before a conjunction like więc when it joins clauses.

Yes. Dziś and dzisiaj both mean today.

• dziś is shorter
• dzisiaj is slightly fuller in form

In most situations, they are interchangeable. Dziś is very common and natural.

Odpoczywa is the 3rd person singular present tense of odpoczywać.

It means:

• she rests
• she is resting

This is important: Polish present tense often covers both the English simple present and present continuous.

So dziś odpoczywa can naturally mean today she is resting.

Also, odpoczywać is imperfective, which fits an ongoing or temporary action.

Because w means in here, and when it expresses location, it normally takes the locative case.

• dom = house/home
• w domu = at home / in the house

So:

• w domu = location
• compare do domu = to home / homeward (movement toward home)

The form domu here is the locative singular.

Yes. Polish word order is more flexible than English word order.

This sentence could be rearranged in different ways, for example:

• Dziś moja siostra odpoczywa w domu, bo cierpi na ból głowy.
• Moja siostra dziś odpoczywa w domu, więc cierpi na ból głowy.
  (This second version changes the logic and is not the same meaning, so word order flexibility does not mean everything is interchangeable.)

In your original sentence, the order is natural:

• first the topic: moja siostra
• then the reason: cierpi na ból głowy
• then the result: więc dziś odpoczywa w domu

Changing word order usually changes emphasis, not the core grammar.

A few useful pronunciation points:

• siostra: roughly SHO-stra, but the first sound is softer than English sh
• cierpi: roughly CHER-pee, with a soft first consonant
• głowy: roughly GWO-vih
  
  • the letter ł sounds like English w
• dziś: roughly jeesh, but shorter and softer than that English approximation
• więc: roughly vyents, with a nasal vowel effect before the final consonant

A few key letter facts:

• ł = English w
• ś = a soft sh-like sound
• ci before a vowel often makes a soft ć sound
• dzi often gives a soft j/dzh-like sound

English approximations are only rough guides, but they help at the start.