Learn Hardcore Czech: Já znám tvoji sestru, ale ona mě nezná. - I know your sister, but she doesn’t know me.

Znám tvoji sestru is the most typical, neutral form.
Adding Já makes the subject emphatic: I know your sister (but maybe someone else doesn’t) or I know your sister, but….

So Já is not grammatically necessary; it is used for emphasis or contrast.

Why is it znám and not vím? Don’t both mean to know?

Czech has two main verbs for to know:

znát – to know a person, place, or thing (to be acquainted with it)
- Znám tvoji sestru. = I know your sister.
vědět – to know a fact, information
- Vím, kolik je hodin. = I know what time it is.

Because the sentence is about being acquainted with a person, znát is correct: Já znám tvoji sestru, not Já vím tvoji sestru.

Why is it tvoji sestru and not tvá sestra?

Two things are going on: agreement and case.

The basic form is tvá sestra (nominative, your sister as the subject).
Here, your sister is the object of I know, so Czech uses the accusative case:
- sestra → sestru (feminine animate/inanimate accusative)
- tvá → tvoji (possessive agreeing with feminine accusative singular)

So the object form must be tvoji sestru, not tvá sestra.

Where does the form tvoji come from? What are the forms of tvůj?

Tvůj is the informal singular your and it changes for gender and case. For the most common nominative forms:

masculine: tvůj bratr – your brother
feminine: tvá / tvoje sestra – your sister
neuter: tvoje auto – your car

In the accusative feminine singular, tvá / tvoje becomes:

tvoji / tvou (both exist; tvoji is more common in everyday speech)

So tvoji sestru is your sister in the accusative.

Why is it sestru and not sestra?

Sestra is a feminine noun. In this sentence, your sister is the direct object of znát (to know), so it must be in the accusative case.

For many feminine nouns ending in -a, the accusative singular ends in -u:

sestra (nominative) → sestru (accusative)
máma → mámu
kolega (m. person) → kolegu

So znám (koho?) tvoji sestru – I know whom? your sister.

Could I say Znám sestru tvoji or does tvoji have to come before sestru?

The neutral word order places the possessive before the noun:

Znám tvoji sestru. – natural, standard

Znám sestru tvoji is possible but sounds strongly emphatic or poetic and is unusual in everyday speech. It would put stress on tvoji, almost like that sister of yours (as opposed to someone else’s). For normal conversation, use tvoji sestru.

Why is it ona mě nezná and not just mě nezná or nezná mě? Where do pronouns go?

Czech word order is flexible but has preferences:

ona mě nezná – explicit subject first (emphasis: she doesn’t know me)
mě nezná – starts with mě, which is unusual in isolation and sounds strongly focused (me, she doesn’t know).
nezná mě – also correct and quite natural: Nezná mě. (She doesn’t know me.)

Subject pronouns like ona are often dropped if context is clear:

Znám tvoji sestru, ale nezná mě. = also fine and common.

Here ona is used for contrast: Já znám… ale ona…

Why is it mě and not mně?

Czech distinguishes mě and mně, both pronounced [mňe] in standard speech, but used in different cases:

mě – accusative and genitive (most of the time)
mně – dative and locative (and in a few fixed expressions)

Here the pronoun is a direct object: she doesn’t know *me (whom?) → *accusative, so we use mě:

ona mě nezná – she doesn’t know me
Compare:
Dělá to pro mně. – He does it for me. (dative → mně)

How is negation with nezná formed? Where does the ne- come from?

Czech forms verbal negation mostly by adding the prefix ne- in front of the verb:

zná → nezná – he/she/it knows → doesn’t know
mám → nemám – I have → I don’t have
chápu → nechápu – I understand → I don’t understand

So ona mě nezná is literally she me not-knows. The ne- prefix is attached directly to the verb; it’s not written as a separate word.

Could I change the word order to Tvoji sestru znám, ale ona mě nezná? Does the meaning change?

Yes, that word order is correct and commonly used. The basic meaning is the same, but the focus changes slightly:

Já znám tvoji sestru… – more neutral, light emphasis on I
Tvoji sestru znám… – emphasizes your sister (as opposed to someone else), like Your sister I know, but she doesn’t know me.

Czech uses word order a lot to indicate information structure (what is new or emphasized), not just grammar.

Is tvoji informal? How would I say this politely or formally?

Yes, tvůj / tvoje / tvoji are informal singular – used to address:

friends
relatives
people your age or younger in casual contexts.

For formal or plural “you”, use váš:

Znám vaši sestru, ale ona mě nezná. – I know your (formal/plural) sister, but she doesn’t know me.

So:

informal: tvoji sestru
formal / plural: vaši sestru

How do I pronounce Já znám tvoji sestru, ale ona mě nezná? Where is the stress?

Key points:

Stress in Czech is always on the first syllable of each word:
Já znám tvo-ji ses-tru, a-le o-na mě nez-ná.
Long vowels are marked with a ˉ:
- á in Já, znám, nezná are long.
ě in mě is pronounced like m-nye ([mňe]) in standard Czech.

Very roughly in English-like transcription (not perfect):

Já – yaah
znám – znaam (long a)
tvoji – tvo-yi
sestru – ses-troo
ale – a-leh
ona – o-na
mě – mn-yeh
nezná – nez-naah

All words have stress on their first syllable.