Learn Hardcore Czech: Moje sestra má lepší náladu, když cvičí doma a běhá pomalu v parku. - My sister is in a better mood when she exercises at home and runs slowly in the park.

It still behaves like an adjective and must agree in case, number and gender with the noun nálada (feminine, accusative singular), but its form happens to be lepší in that slot:

nominative sg: lepší nálada
accusative sg: lepší náladu

Why is there a comma before když?

In Czech, a subordinate clause introduced by když (“when, whenever”) is normally separated by a comma from the main clause.

Main clause: Moje sestra má lepší náladu
Subordinate clause: když cvičí doma a běhá pomalu v parku.

So you write:

Moje sestra má lepší náladu, když cvičí doma a běhá pomalu v parku.

Omitting the comma here would be considered a spelling/punctuation mistake in standard written Czech.

Why is it když and not kdy?

když = “when / whenever” as a conjunction introducing a clause.
kdy = “when?” as a question word.

In this sentence you are not asking a question, you are introducing a condition/time:

… má lepší náladu, když cvičí… = “… is in a better mood when she exercises…”

Using kdy here would be wrong:

❌ Moje sestra má lepší náladu, kdy cvičí…

Why is the subject she not repeated in the second part? Why not když ona cvičí?

Czech usually omits subject pronouns (já, ty, on, ona, etc.) when the subject is clear from context or from the verb ending.

We already know the subject is moje sestra.
The verb form cvičí can refer to “he/she/it exercises” (3rd person singular).
So když cvičí doma naturally refers back to moje sestra.

You could say když ona cvičí, but that sounds marked/emphatic, like stressing she in English (“when she exercises”). For a neutral sentence, když cvičí is correct and natural.

Why is it moje sestra instead of má sestra? Don’t both mean “my sister”?

Both moje and má mean “my” in the feminine singular nominative:

moje sestra = my sister
má sestra = my sister (slightly more formal / written style)

Differences:

moje is the full form, very common in spoken language and neutral style.
má is a shorter form, more typical in written or more formal contexts, and sometimes used to adjust rhythm in poetry or elevated speech.

In everyday neutral sentences, moje sestra is more common and perfectly natural, which is why it’s used here.

What’s the nuance of cvičí vs something like zacvičí si?

The verb here is cvičit (imperfective), present tense cvičí = “(she) exercises / works out”.

cvičí describes a general, habitual activity:
- když cvičí doma = when she exercises (in general) at home.

If you said zacvičí si, that’s perfective and more like:

když si zacvičí doma = when she has a (one-time) workout at home / when she manages to exercise at home.

In your sentence, the point is about what she typically does, so the simple imperfective cvičí is the right choice.

Why is it běhá and not běží?

Czech distinguishes two common verbs for “to run”:

běhat (imperfective, iterative/habitual)
běžet (imperfective, but usually one concrete run / movement “is running”)

Present forms:

běhá = runs (as a habit, repeatedly)
běží = is running (right now) / is in the act of running

In your sentence, we talk about usual behavior:

… když cvičí doma a běhá pomalu v parku.
= when she works out at home and (regularly) runs slowly in the park.

So běhá (habitual) fits better than běží (one specific run).

Why is doma used without a preposition, but v parku has v?

Doma is an adverb meaning “at home”; it does not take a preposition:

cvičí doma = she exercises at home

By contrast, park is a noun, and to express a static location “in the park” you use the preposition v + locative case:

v parku = in the park (park → v parku)

So:

doma = at home (adverb, no preposition)
v parku = in the park (preposition + noun in locative)

Why is the adverb pomalu placed before v parku? Can we change the word order?

Standard neutral Czech word order for adverbials is often:

verb – manner – place – (time)

So:

běhá pomalu v parku
= runs slowly in the park.

Other orders are possible, but they can sound more marked or shift the emphasis:

běhá v parku pomalu – more emphasis on pomalu (“it’s in the park that she runs slowly”).
pomalu běhá v parku – stylistically less neutral, stressing that the whole running is slow.

Your original order (běhá pomalu v parku) is the most neutral and natural.

What tense is cvičí and běhá, and does it mean “is exercising / is running” or “exercises / runs”?

Both cvičí and běhá are in the present tense.

In Czech, the present tense of imperfective verbs can express:

Right-now actions (like English “is doing”), or
General/habitual actions (like English “does regularly”).

Here the meaning is habitual:

když cvičí doma = when she exercises (in general) at home
a běhá pomalu v parku = and runs slowly (regularly) in the park

So in natural English you’d translate the whole sentence as:

My sister is in a better mood when she works out at home and runs slowly in the park.

even though the Czech form could literally also match “when she is exercising and running…”. Context decides the nuance.