Learn Hardcore Czech: Moje sestra občas odpočívá v parku a poslouchá hudbu. - My sister sometimes relaxes in the park and listens to music.

In everyday spoken Czech, moje sestra is more common and neutral.
Má sestra sounds a bit more literary or formal, but you will definitely see it in books, newspapers, etc.

Why is there no word for “is” like in English “My sister is resting…”?

Czech normally does not use “být” (to be) to form continuous tenses the way English does.

English: is resting, is listening (present continuous)
Czech: just odpočívá, poslouchá (simple present, which covers both “rests” and “is resting”)

So:

Moje sestra odpočívá = My sister rests / My sister is resting
You do not say: Moje sestra *je odpočívá* – that is incorrect.

Where can občas go in the sentence? Is its position flexible?

občas means sometimes / occasionally, and it’s fairly flexible in word order.

All of these are possible and natural, with only small changes in emphasis:

Moje sestra občas odpočívá v parku a poslouchá hudbu.
Občas moje sestra odpočívá v parku a poslouchá hudbu.
Moje sestra odpočívá občas v parku a poslouchá hudbu. (emphasis that the park is where she sometimes rests)

The version you have, Moje sestra občas odpočívá…, is probably the most typical for neutral speech: subject → frequency adverb → verb.

Why is it v parku and not v park or do parku?

The preposition decides the case of the noun:

v (in) + locative case → v parku = in the park
do (to/into) + genitive case → do parku = to the park / into the park

Here we are talking about location (where she is resting), so we use v + locative:

Kde odpočívá? – V parku. (Where does she rest? – In the park.)

You could say:

Moje sestra chodí do parku. – My sister goes (walks) to the park. (movement towards)

Why does park change to parku? Where does the -u ending come from?

park is a masculine inanimate noun. In the locative singular (used after v = in, na = on, etc. for locations), masculine nouns often take the ending -u:

park → v parku (in the park)
les (forest) → v lese (in the forest)
hotel → v hotelu (in the hotel)

So parku here is locative singular, required by v when it means in.

Why is hudbu used instead of hudba?

hudba is a feminine noun meaning music.

In this sentence, hudbu is the direct object of poslouchá (listens to), so it must be in the accusative case:

Nominative (dictionary form, subject): hudba
Accusative (direct object): hudbu

Examples:

Hudba je hlasitá. – The music is loud. (subject → nominative)
Poslouchá hudbu. – She listens to music. (object → accusative)

Why is the subject “my sister” mentioned only once, before the first verb? How do we know it also belongs to poslouchá?

In Czech, when two verbs share the same subject and are joined with a (and), you typically state the subject only once, at the beginning:

Moje sestra občas odpočívá v parku a poslouchá hudbu.
= My sister sometimes rests in the park and (she) listens to music.

The subject moje sestra logically applies to both verbs:

(Moje sestra) odpočívá…
(Moje sestra) poslouchá…

You could repeat it:

Moje sestra občas odpočívá v parku a moje sestra poslouchá hudbu.
…but this sounds unnatural and heavy in Czech.

What is the difference between odpočívá and odpočine si / odpočinout si?

These are different aspects of the verb “to rest”:

odpočívat → odpočívá
- imperfective
- focuses on the process or a habit
- can express “is resting” / “rests (habitually)”
odpočinout si → odpočine si
- perfective
- focuses on the completed act of resting, the result (“to have a rest”)
- often used for one-time, complete events

In your sentence we describe a habitual action, so we use the imperfective:

Moje sestra občas odpočívá v parku…

If you talk about a specific occasion:

Moje sestra si odpočine v parku a pak půjde domů.
– My sister will have a rest in the park and then go home.

Can I change the order of the two actions? For example:
Moje sestra občas poslouchá hudbu a odpočívá v parku.

Yes, that sentence is grammatically correct and understandable.

However, the focus changes slightly:

…odpočívá v parku a poslouchá hudbu.
→ You first present where she rests and then add what she does while resting.
…poslouchá hudbu a odpočívá v parku.
→ You first say she listens to music, and only then add that she rests in the park.

In practice, both orders can be used, and context will usually make the meaning clear.

What is the difference between občas and někdy?

Both relate to “sometimes”, but there is a nuance:

občas – once in a while, occasionally
- Suggests not very often.
někdy – can mean:
- sometimes (similar to English “sometimes”), or
- ever / at some time (e.g. “sometime in the future”)

In your sentence, both are possible:

Moje sestra občas odpočívá v parku… (neutral “sometimes, occasionally”)
Moje sestra někdy odpočívá v parku… (also “sometimes”, slightly broader use)

Context usually decides which is more natural, but občas is a nice straightforward “occasionally”.

Could I use relaxuje instead of odpočívá?

You can say:

Moje sestra občas relaxuje v parku a poslouchá hudbu.

relaxovat → relaxuje is a loanword from English (“to relax”). It sounds a bit more colloquial / modern, sometimes slightly trendy, but it’s very common in everyday speech.

odpočívat → odpočívá is the native Czech verb and is completely neutral. Both are correct; odpočívá is stylistically a bit more “standard Czech”.

Is it possible to say just poslouchá without an object, like English “She listens”?

Yes, you can say simply:

Poslouchá. – She is listening.

But in Czech, poslouchat very often expects an object: What is she listening to? So poslouchá hudbu, poslouchá rádio, poslouchá podcasty, etc., is more typical.

If you want to emphasize that she listens (pays attention) to someone, you can also use naslouchat:

Naslouchá ptákům. – She listens to the birds (attentively).

In your sentence, poslouchá hudbu is the most natural way to say listens to music.