Moje sestra určitě přijde na oslavu.

Both are correct.

• moje sestra = very common and natural in everyday Czech
• má sestra = also correct, but it can sound a bit more formal, literary, or emphatic

So for a learner, moje sestra is a very safe and natural choice.

Also note that má can also mean she has, but in má sestra, the context clearly shows it means my sister.

Sestra is in the nominative singular because it is the subject of the sentence — the person doing the action.

• sestra = sister
• moje sestra = my sister

The verb přijde agrees with this subject.

Formally, přijde looks like a present-tense form, but here it has future meaning.

That is because přijít is a perfective verb. In Czech, perfective verbs use present-tense forms to talk about a completed future action.

So:

• přijde = he/she will come
• not he/she is coming in this sentence

Compare:

• přichází = he/she is coming / comes (imperfective)
• přijde = he/she will come (perfective)

The dictionary form is přijít.

In this sentence:

• přijde = he/she/it will come

Because Czech verbs show person and number in the verb ending, you do not need a separate subject pronoun like she if the subject is already stated.

Určitě means:

• definitely
• certainly
• for sure

It adds confidence or certainty to the statement.

So Moje sestra určitě přijde na oslavu means that the speaker is quite sure their sister will come.

Czech often uses na + accusative for going or coming to events.

So you say:

• na oslavu = to the celebration / to the party
• na koncert = to the concert
• na svatbu = to the wedding

This is just the normal Czech pattern for many events and social occasions.

Using do would not sound natural here.

Because after na meaning movement to an event, Czech uses the accusative case.

The base form is:

• oslava = celebration, party

But in this sentence it changes to:

• na oslavu

So:

• oslava = nominative
• oslavu = accusative

This is a very common feminine noun pattern.

The difference is the case and the meaning:

• na oslavu = to the celebration → movement toward the event
• na oslavě = at the celebration → location at the event

So:

• Moje sestra přijde na oslavu. = My sister will come to the celebration.
• Moje sestra je na oslavě. = My sister is at the celebration.

This is a very useful contrast to learn.

Not completely. Czech word order is more flexible than English, but the sentence as given is a very natural, neutral order:

• Moje sestra určitě přijde na oslavu.

This sounds like a normal statement.

You can move words around for emphasis, for example:

• Určitě moje sestra přijde na oslavu.
• Moje sestra přijde na oslavu určitě.

These versions are possible, but they can slightly change the emphasis. For learners, the original order is the best one to copy first.

Because Czech does not have articles like English a and the.

Czech simply says:

• sestra
• oslava

Whether English uses a or the depends on context and translation.

So na oslavu could be translated as:

• to a celebration
• to the celebration
• to the party

depending on the situation.

The sound ř is one of the hardest Czech sounds for English speakers.

A simple learner-friendly way to think about it:

• it is somewhat like a rolled/trilled r
• combined with a zh-like sound

You do not need to master it immediately to be understood. A plain r is often an acceptable learner approximation at first.

So for beginners:

• určitě: try something like oor-chi-tye
• přijde: try something like przhiy-de or a simplified pri-yde

Those are only rough guides, but they can help you get started.