Learn Hardcore Czech: Moje sestra běhá v parku rychleji než já. - My sister runs in the park faster than I do.

běhat – repeated, habitual, “generally runs”
- present: běhám, běháš, běhá…
běžet – one specific movement, “is running / runs once”
- present: běžím, běžíš, běží…

In Moje sestra běhá v parku rychleji než já, we are talking about her general ability or habit (she usually runs faster than I do), so the habitual verb běhat is used: běhá.

If you meant right now, in this particular moment, you would more likely say:
Moje sestra běží v parku rychleji než já.

What is the difference between moje sestra and má sestra?

Both mean my sister.

moje sestra – neutral, very common in everyday speech.
má sestra – a shorter, slightly more formal/literary variant.

In normal conversation, moje sestra is more typical. In written style, literature, or more elevated speech, you will also see má sestra.

Why is it rychleji and not rychlejší?

It depends on what you are comparing:

rychlejší is the comparative of the adjective rychlý (fast). It describes a noun:
- Je rychlejší běžec. – He is a faster runner.
rychleji is the comparative of the adverb rychle (fast). It describes a verb (how someone runs):
- Běhá rychleji. – She runs faster.

In the sentence, we are talking about how she runs (modifying běhá), so we need the adverb: rychleji.

How is the form rychleji formed? Is there a rule?

Yes. Many adverbs in Czech form their comparative with -eji / -ěji:

rychle → rychleji (fast → faster)
pomalu → pomaleji (slowly → more slowly)
brzy → dříve (soon → sooner; this one is irregular)

Often, the base adverb comes from an adjective in -ý:

rychlý (fast, adj.) → rychle (fast, adv.) → rychleji (faster, adv.)

So in the sentence we use rychleji because it is the correct comparative adverb.

Why is it než já? Can I also say než mě?

After než (“than”) with a comparative, the most standard form is the nominative pronoun, because an implied verb follows:

…rychleji než já (běhám). – faster than I (run).

So než já is the textbook, grammatically “correct” choice.

In everyday spoken Czech, you will also very often hear než mě (accusative) or než mně (dative), even though traditional grammar labels them as non‑standard here. Native speakers use než mě a lot, but if you want to be safe for exams or formal writing, use než já.

Do I need to repeat the verb after než já? For example …rychleji než já běhám?

No, it’s not necessary, and usually it is not repeated.

Natural: Moje sestra běhá v parku rychleji než já.
(The verb běhám is understood after já.)
Possible but usually only for emphasis:
- Moje sestra běhá v parku rychleji než běhám já.
  (Strong emphasis on já as the person being compared.)

Simply …rychleji než já is the normal, neutral way.

Why is it v parku with -u at the end? What case is that?

V parku is preposition + locative case:

Preposition v (“in”) + locative singular of park = v parku.

Czech uses:

v
- locative for location:
  - v parku – in the park
  - v domě – in the house
Different prepositions/cases for movement towards a place, e.g.:
- do parku – into the park (preposition do
  - genitive)
- na hřiště – onto the playground (preposition na
  - accusative)

In our sentence, we’re describing where she runs (location), so v parku (locative) is correct.

Can I change the word order in this sentence? For example, Moje sestra v parku běhá rychleji než já?

Yes, Czech word order is flexible, and you can move parts around to change the emphasis, as long as you keep natural groupings.

All of these are acceptable:

Moje sestra běhá v parku rychleji než já. – neutral.
V parku běhá moje sestra rychleji než já. – emphasis on the place “in the park”.
Moje sestra běhá rychleji v parku než já. – slight emphasis that in the park she runs faster than I do (could imply comparison to other places).

You usually keep rychleji než já together as a unit. Variants like
Moje sestra v parku rychleji běhá než já sound awkward to native speakers.

Could I drop moje and just say Sestra běhá v parku rychleji než já?

Yes, that is possible.

Czech often omits possessive pronouns when the possessor is clear from context (especially with close family members):

Sestra běhá v parku rychleji než já.

This normally still means my sister if we are talking about my family. If you want to be explicit or are introducing the person for the first time, Moje sestra is clearer.

How would I say “My sister is running in the park faster than I am (right now)”? Is the same sentence used?

Czech does not have a separate continuous tense, so the present tense covers both:

My sister runs… (habitually)
My sister is running… (right now)

To show that you mean “right now”, you usually add a time word and pick the “single action” verb:

Moje sestra teď běží v parku rychleji než já.
– My sister is running in the park faster than I am (now).

In your original sentence with běhá, the default reading is more habitual: she generally runs faster than I do.

Why don’t we say víc rychle or více rychle for “more fast”?

Czech usually forms the comparative of basic adjectives and adverbs with special comparative forms, not with víc(e):

rychlý / rychle → rychlejší / rychleji (fast → faster)
pomalu → pomaleji (slowly → more slowly)

Using víc rychle would sound unnatural. Víc / více is used mainly with:

longer or less common adjectives/adverbs:
- efektivně → víc efektivně (more effectively)
verbs or nouns:
- Více běhá. – He runs more (more often / more in quantity).
- Více peněz. – More money.

Why is it sestra and not sestru?

Sestra is in the nominative case because it is the subject of the verb běhá:

Kdo běhá? – Moje sestra. (Who runs? My sister.) → nominative sestra

You would use sestru (accusative) when it is the object:

Vidím svoji sestru. – I see my sister.
Mám jen jednu sestru. – I have only one sister.

In your sentence, your sister is the one doing the action, so the nominative form sestra is correct.