Learn Hardcore Czech: Šaty, které si chce vzít moje sestra na večírek, jsou bílé a velmi elegantní. - The dress that my sister wants to put on for the party is white and very elegant.

Antecedent: šaty – plural, masculine inanimate (by grammar, even if it’s a dress)
Function in the clause: direct object of vzít si (“to take/put on”)
Case: accusative (because you take something)

For šaty as a plural masculine inanimate noun:

Nominative plural: které
Accusative plural: které

So které here is accusative plural, matching šaty (“the dress/clothes that my sister wants to wear…”).

What does si chce vzít mean, and why do we need si?

The verb here is vzít si. Literally:

vzít = to take
vzít si (na sebe) = to take for oneself / put on (clothes), i.e. “to wear” for an occasion

So chce si vzít šaty na večírek ≈ “she wants to wear the dress to the party.”

The little word si is a reflexive pronoun. With clothing, it usually means:

vzít si šaty – put the dress on / wear it
vzít šaty (without si) – take/pick up the dress (e.g. from a hanger), not necessarily put it on

In this sentence, we are clearly talking about wearing the dress, so si is necessary:
které si chce vzít moje sestra na večírek = “which my sister wants to wear to the party.”

Why is the word order které si chce vzít moje sestra, and can I say které chce si vzít moje sestra or které moje sestra si chce vzít?

Czech has relatively free word order, but clitics (short, unstressed words like si) have strong placement rules.

General rule:
Clitics like si usually stand in the second position in the clause (after the first stressed word or phrase).

In the clause:

které si chce vzít moje sestra

the order is:

které – first stressed word
si – clitic in second position
chce vzít moje sestra – the rest

This is very natural Czech.

Other versions:

které moje sestra si chce vzít – possible, but sounds marked/emphatic; normal speech would more often put moje sestra at the start of the clause:
Moje sestra si chce vzít tyto šaty na večírek.
které chce si vzít moje sestra – sounds wrong/unnatural, because si is too far from the beginning; native speakers generally avoid this order.

So: které si chce vzít moje sestra is the standard, natural order here.

Why na večírek and not do večírku? How does na work here?

Czech often uses na + accusative with events and activities:

na večírek – to a party
na koncert – to a concert
na oběd – to (for) lunch
na dovolenou – on holiday
na schůzku – to a meeting/date

So na večírek means “to the party” in the sense of attending an event.

Do is more about going into a physical space:

do školy – to (into) school
do města – to (into) town
do obchodu – to (into) a shop

A party is treated as an event, not so much as a container-like space, so Czech naturally uses na večírek, not do večírku.

Why are there commas around které si chce vzít moje sestra na večírek?

In Czech, subordinate clauses introduced by který / která / které (relative clauses) are normally separated by commas from the main clause.

Here, the structure is:

Šaty, [které si chce vzít moje sestra na večírek], jsou bílé a velmi elegantní.

So we enclose the whole relative clause in commas.

Functionally, in English this is similar to:

“The dress, which my sister wants to wear to the party, is white and very elegant.”

Czech does not distinguish with or without commas the way English does with “which” vs “that”; you basically always add the comma(s) around a který/která/které clause like this.

Why is it bílé šaty and not bílá šaty or bílý šaty?

Adjectives in Czech must agree with the noun in gender, number, and case.

Noun: šaty – plural, masculine inanimate (grammatically)
Case here: nominative plural (subject of the verb jsou)

The nominative plural endings:

masculine inanimate plural: dobré stoly
feminine plural: dobré ženy
neuter plural: dobrá auta

So with šaty you use the plural ending -é:

bílé šaty – white dress/clothes
krátké šaty – short dress
letní šaty – summer dress

Therefore the sentence has:

Šaty … jsou bílé … – both šaty and bílé are plural.

Why is elegantní the same form as in the singular? Is it not changing?

Many adjectives in Czech ending in -ní / -ní (often from foreign words) have the same form in nominative singular and plural:

elegantní šaty – elegant dress (plural noun)
elegantní šaty jsou… – elegant clothes are…
elegantní muž – elegant man
elegantní žena – elegant woman
elegantní auto – elegant car

So in nominative they all look like elegantní, regardless of gender or number.

That does not mean they never change; in other cases you get forms like:

bez elegantního muže – without an elegant man
k elegantním šatům – to/with the elegant dress/clothes

In the sentence Šaty … jsou bílé a velmi elegantní, the adjective elegantní just happens to be in a form that looks the same in singular and plural.

Is moje sestra different from má sestra? Which one is better here?

Both mean “my sister”:

moje sestra – neutral, everyday, very common
má sestra – a bit more formal, bookish, or used where a shorter form sounds smoother

In this sentence:

které si chce vzít moje sestra na večírek
and
které si chce vzít má sestra na večírek

are both grammatically correct. Most speakers would naturally say moje sestra in normal conversation. Má sestra might sound slightly more literary or careful.

Why is moje sestra placed after chce vzít? Could I say Moje sestra si chce vzít šaty, které… instead?

The original order:

Šaty, které si chce vzít moje sestra na večírek, jsou…

starts with Šaty as the main topic and then adds information about them.

You can absolutely rephrase with the sister as the topic:

Moje sestra si chce vzít na večírek šaty, které jsou bílé a velmi elegantní.

Moje sestra si chce vzít šaty na večírek; jsou bílé a velmi elegantní.

Czech word order is flexible and often used to change focus or emphasis:

Original: focuses more on the dress (what it’s like).
Rephrased: focuses more on the sister and her intention.

In the relative clause které si chce vzít moje sestra, putting moje sestra towards the end is very natural; the new information (the subject) can come later in the clause.

Why is velmi used here, and how is it different from moc, hodně, or strašně?

Velmi is a neutral, slightly more formal adverb meaning “very”:

velmi elegantní – very elegant

Other common intensifiers:

moc elegantní – very/really elegant (more colloquial)
hodně elegantní – very/a lot elegant (colloquial; sounds a bit less “refined” stylistically)
strašně elegantní – “terribly” elegant (very colloquial, expressive)

So:

velmi elegantní – good in writing, formal speech, neutral style
moc / hodně / strašně elegantní – frequent in everyday informal conversation

In your sentence, velmi elegantní fits well with the relatively neutral, slightly careful style.