Learn Hardcore Czech: Nevím, jestli dnes večer přijde moje sestra. - I don’t know if my sister will come this evening.

In Czech, a comma is written before almost every subordinate clause, and jestli introduces a subordinate clause (an indirect if/whether question).

Nevím, ← main clause: I don’t know
jestli dnes večer přijde moje sestra. ← subordinate clause: if my sister will come this evening

Because jestli starts the subordinate clause, standard Czech spelling requires a comma before it: Nevím, jestli ….
So the comma is a normal grammar rule, not a pause marker like in English.

Why do we use jestli here? Could we also say zda? What is the difference?

Both jestli and zda can mean if / whether in indirect yes/no questions, and both are possible here:

Nevím, jestli dnes večer přijde moje sestra.
Nevím, zda dnes večer přijde moje sestra.

The difference:

jestli – neutral, very common in everyday spoken Czech and in informal writing.
zda – more formal, typical of written language, official texts, news, academic style.

Meaning is the same; the choice is mostly about style/level of formality.

Why is it přijde and not something like bude přijít to talk about the future?

Czech does not form the future of most verbs the way English does. Instead, it uses aspect:

přijít – perfective verb (“to come, to arrive” as a single completed event)
its present forms (e.g. přijdu, přijdeš, přijde) usually refer to the future.

So přijde here means will come.

You cannot say ✗ bude přijít – that is ungrammatical.

If you used the imperfective verb přicházet (“to be coming / to come repeatedly”), then the future would be bude přicházet, but that would describe an ongoing or repeated coming, not one arrival this evening. For a single visit tonight, přijde is correct.

Can I change the word order and say Nevím, jestli moje sestra přijde dnes večer? Does it change the meaning?

Yes, that sentence is correct too:

Nevím, jestli dnes večer přijde moje sestra.
Nevím, jestli moje sestra přijde dnes večer.

Both basically mean the same: I don’t know if my sister will come this evening.

Czech word order is flexible. Differences:

jestli dnes večer přijde moje sestra – slightly stronger focus on přijde moje sestra as a whole (the event of her coming).
jestli moje sestra přijde dnes večer – slightly stronger focus on dnes večer (when she will come).

In everyday speech, both sound natural; the nuance is very small.

Why is it moje sestra and not má sestra? Are both correct?

Both are grammatically correct:

moje sestra
má sestra

They are two forms of the same possessive adjective můj / moje / mé (my). In the feminine singular nominative, you can use either moje or the shorter form má.

Differences:

moje sestra – neutral, very common in everyday speech.
má sestra – a bit more formal/literary, or used for rhythm/emphasis.

In spoken Czech most people will say moje sestra.

Why isn’t it svá sestra instead of moje sestra? Isn’t svůj the reflexive my?

The reflexive possessive svůj / svoje is used when the possessor is the subject of the same clause.

Here, in the subordinate clause, the subject is moje sestra (my sister), but the possessor is I (from the main clause nevím = I don’t know). So possessor and subject are not the same within that clause.

Therefore you must use moje sestra, not svá sestra.
svůj would be used if the sister owned something and was herself the subject of that clause, e.g.:

Moje sestra ví, kde je svůj pas.
My sister knows where her (own) passport is.

Why is there no já before nevím? How do we know it means I don’t know?

Czech usually drops subject pronouns when the verb ending already shows the person:

vím = I know
víš = you (sg.) know
ví = he/she/it/they know
víme = we know

Because vím clearly means I know, nevím (with ne- for negation) clearly means I don’t know, so já is unnecessary:

Nevím, jestli… – natural, normal.
Já nevím, jestli… – also correct, but used for emphasis (I don’t know) or in more emotional speech.

So the default is to leave já out when it’s obvious.

What exactly does dnes večer mean? Could I just say večer?

dnes = today
večer = evening / in the evening

Together, dnes večer means this evening / tonight (this evening).

You can say:

Nevím, jestli večer přijde moje sestra.
= I don’t know if my sister will come in the evening (today is usually understood from context).

The phrase dnes večer is very common and sounds completely natural; it just makes the time explicit.

What case is večer in, and why is there no preposition like v?

Formally, večer here has the same form as the accusative singular of the noun večer. However, in practice you can just think of it as a set phrase used as an adverbial of time: in the evening.

Czech often uses bare time nouns without a preposition:

včera – yesterday
dnes – today
zítra – tomorrow
ráno – in the morning
večer – in the evening

So you don’t say ✗ v večer, just večer (or dnes večer).

Can I leave out the possessive and say Nevím, jestli dnes večer přijde sestra?

Yes, that is grammatically correct: Nevím, jestli dnes večer přijde sestra.

However, the meaning changes:

moje sestra – clearly my sister.
sestra on its own can mean:
- a (particular) sister already known from context, or
- a nurse (in some contexts, like in a hospital).

So if you specifically mean my sister, you normally keep moje. Omitting it is only natural if the context already makes it totally clear whose sister you are talking about.

What is the difference between jestli and když? Both can be translated as if.

Both can translate as if, but they are not interchangeable.

jestli introduces indirect questions and real conditions similar to if / whether:
- Nevím, jestli dnes večer přijde moje sestra.
  I don’t know if my sister will come this evening.
- Jestli přijde, budu rád.
  If she comes, I’ll be glad.
když is more like when / whenever / if (whenever):
- Když přijde, dá mi vědět.
  When(ever) she comes, she lets me know.
- Když je hezky, jdeme ven.
  When / If it’s nice, we go outside.

In your sentence you are expressing uncertainty (you don’t know whether she’ll come), so jestli is the natural choice; když would be wrong here.

Is přijde about coming on foot? What if she comes by car?

Přijde is from přijít, which is mainly to come (on foot), but in everyday speech it is often used more generally for to come / to show up, regardless of transport, especially for relatively short distances.

If you specifically want to say come/arrive by vehicle, you can use přijet:

Nevím, jestli dnes večer přijede moje sestra.
I don’t know if my sister will arrive (by car/train/bus) this evening.

Both are possible; choice depends on what you want to emphasize.