Ten kostium jest piękny, ale mój występ zaczyna się dopiero wieczorem.

Because kostium is a masculine singular noun, so the demonstrative ten must match it.

In Polish, this changes depending on the gender of the noun:

• ten for masculine nouns
• ta for feminine nouns
• to for neuter nouns

So:

• ten kostium = this costume
• ta sukienka = this dress
• to ubranie = this clothing/item of clothing

Here, kostium is masculine, so ten is correct.

The adjective has to agree with the noun it describes.

Since kostium is masculine singular, the adjective also takes the masculine singular form:

• piękny kostium = a beautiful costume

Compare:

• piękna sukienka = a beautiful dress
• piękne ubranie = beautiful clothing / a beautiful garment

So piękny matches kostium.

For the same reason: występ is also a masculine singular noun, so the possessive my must match it.

Polish forms of my include:

• mój for masculine singular
• moja for feminine singular
• moje for neuter singular and some plural uses

So:

• mój występ = my performance
• moja rola = my role
• moje zadanie = my task

Because występ is masculine, mój is the correct form.

Występ usually means a performance, especially one on stage, in public, or as part of a show.

It can refer to:

• a theatrical performance
• a dance or music performance
• a public appearance in a show or event

So in this sentence, mój występ means my performance.

Zaczyna się means begins or starts.

The verb is zaczynać się / zacząć się, which is a very common Polish way to say that something starts:

• Film zaczyna się o ósmej. = The film starts at eight.
• Koncert zaczyna się jutro. = The concert starts tomorrow.

The się here is part of the verb expression. In English, we just say starts, but in Polish many verbs commonly appear with się.

So:

• Mój występ zaczyna się... = My performance starts...

You should learn zaczynać się as a full unit.

Yes, się is often placed after the verb, and that is what you see here:

• zaczyna się

This is a very normal word order.

In some sentences, się can move, especially in longer clauses, but for learners the safest pattern is usually:

• verb + się

So mój występ zaczyna się dopiero wieczorem is a natural standard sentence.

Dopiero adds the idea of only, not until, or as late as.

So:

• zaczyna się wieczorem = it starts in the evening
• zaczyna się dopiero wieczorem = it doesn’t start until the evening / it only starts in the evening

It suggests that the speaker thinks the time is relatively late, or that something happens later than you might expect.

In this sentence, it creates the contrast:

• The costume is beautiful, but my performance doesn’t start until the evening.

Because Polish often uses the instrumental case to express times of day in an adverb-like way.

So:

• rano = in the morning
• wieczorem = in the evening
• nocą = at night

Here, wieczorem means in the evening.

The base noun is wieczór = evening, but after changing form for this time expression, it becomes wieczorem.

This is something worth memorizing as a set phrase.

Normally, no. For this meaning, Polish usually says wieczorem.

So:

• Mój występ zaczyna się wieczorem. = natural
• Mój występ zaczyna się w wieczór. = unnatural in normal Polish

Use wieczorem for in the evening.

Because ale means but, and in Polish it normally introduces a new clause. A comma is required before it.

So:

• Ten kostium jest piękny, ale mój występ zaczyna się dopiero wieczorem.

This is standard Polish punctuation.

The sentence you have is very natural, but Polish word order is more flexible than English word order.

The given sentence is neutral and clear:

• Ten kostium jest piękny, ale mój występ zaczyna się dopiero wieczorem.

You could rearrange parts for emphasis, for example:

• Ten kostium jest piękny, ale dopiero wieczorem zaczyna się mój występ.

That is possible, but it sounds more marked or stylistically shaped. For learners, the original order is the safest choice.

In this sentence, yes, jest is the normal form.

• Ten kostium jest piękny. = This costume is beautiful.

In very informal speech or in headlines, Polish sometimes drops forms of to be, but in standard full sentences like this one, jest should be used.

So for learners, treat jest here as necessary.

A few useful pronunciation notes:

• piękny: the ę is nasal, but before k it is often pronounced more like pieńkny or pieŋkny in real speech
• występ: the ę before p is also nasalized, often sounding close to wystemp
• się: usually sounds roughly like shyę or śę, with a soft ś sound

Also:

• rz in wieczorem sounds like ż
• cz sounds like English ch in chop, but harder

If you want to sound natural, it is best to listen to these words several times rather than relying only on spelling.

Both are in the nominative case.

That is because they are the main nouns in their clauses:

• Ten kostium jest piękny
• mój występ zaczyna się dopiero wieczorem

In both parts, the noun is the subject of the clause, so nominative is expected.

That is why you get:

• ten kostium
• mój występ

not other case forms.