Kurtyna była jeszcze zamknięta, kiedy widownia zaczęła klaskać.

Because kurtyna is a feminine singular noun in Polish.

In the past tense, the verb być changes to agree with the subject in gender and number:

• był = masculine singular
• była = feminine singular
• było = neuter singular
• byli / były = plural

So:

• Kurtyna była... = The curtain was...

The verb matches kurtyna, not the English idea of it.

For the same reason: it agrees with kurtyna, which is feminine singular.

Zamknięty / zamknięta / zamknięte is a participle/adjectival form meaning closed. It behaves like an adjective, so it changes for gender and number:

• zamknięty = masculine
• zamknięta = feminine
• zamknięte = neuter

So:

• kurtyna była zamknięta = the curtain was closed

This is a very common Polish pattern:

• Drzwi były otwarte = The door was open / the doors were open
• Książka była napisana = The book was written

Polish often expresses a state with być + adjective/participle.

So była zamknięta means the curtain was in the state of being closed.

That is slightly different from focusing on the action of closing it. Compare:

• Kurtyna była zamknięta = The curtain was closed.
  
  • focus: its condition/state
• Kurtynę zamknięto = The curtain was closed.
  
  • focus: the action, with no specific person named

In your sentence, the idea is clearly about the curtain’s state at that moment, so była zamknięta is the natural choice.

Here jeszcze means still.

So:

• Kurtyna była jeszcze zamknięta = The curtain was still closed

It shows that at that moment, the curtain had not opened yet.

This word is very common, but it can mean slightly different things depending on context:

• Jeszcze czekam = I’m still waiting
• Jeszcze nie wiem = I still don’t know / I don’t know yet
• Jeszcze kawałek ciasta? = Another piece of cake?

In this sentence, the meaning is clearly still.

Because widownia is a collective singular noun. It means the audience as one group.

So Polish treats it grammatically as singular:

• widownia zaczęła klaskać = the audience started clapping

That is similar to English the audience when treated as a single unit.

If you wanted to talk about the individual people more directly, you could say something like:

• widzowie zaczęli klaskać = the spectators / audience members started clapping

Notice how the verb becomes plural there: zaczęli.

Because zaczęła klaskać means started clapping, while klaskała means was clapping / used to clap.

This is about aspect and meaning:

• zaczęła klaskać = the beginning of the action is important
• klaskała = the action was ongoing, repeated, or descriptive

So in your sentence:

• widownia zaczęła klaskać = the audience began to clap

That fits the idea of a specific moment: the curtain was still closed when the audience started clapping.

Because Polish commonly uses zacząć + infinitive to mean to start doing something.

So:

• zaczęła klaskać = started to clap / began clapping

Other examples:

• zaczął mówić = he started speaking
• zaczęli biec = they started running
• zaczęłam się śmiać = I started laughing

This is a very normal construction in Polish.

Yes. In this sentence, kiedy and gdy would both work.

• Kurtyna była jeszcze zamknięta, kiedy widownia zaczęła klaskać.
• Kurtyna była jeszcze zamknięta, gdy widownia zaczęła klaskać.

Both mean the same thing here: when.

A small nuance:

• kiedy is very common and neutral
• gdy can sound a little more literary or formal in some contexts, though it is also perfectly normal

For a learner, kiedy is often the safest everyday choice.

Because the whole situation happened in the past.

The sentence describes two past events/states:

1. Kurtyna była jeszcze zamknięta = the curtain was still closed
2. widownia zaczęła klaskać = the audience started clapping

Polish often uses the simple past for both parts, even when English might sometimes use something like was still closed when... started... That is completely natural in Polish.

The relationship in time is shown by the conjunction kiedy and by the meaning of the sentence, not by a special past perfect form.

No, Polish word order is fairly flexible, though some versions sound more natural than others depending on emphasis.

The original sentence is very natural and neutral:

• Kurtyna była jeszcze zamknięta, kiedy widownia zaczęła klaskać.

You could also say:

• Kiedy widownia zaczęła klaskać, kurtyna była jeszcze zamknięta.

This puts more focus on the time clause first: When the audience started clapping...

You can also move jeszcze around a bit, but not every position sounds equally natural. The original placement is excellent for a learner to copy.

A rough pronunciation guide:

• widownia ≈ vee-DOVN-ya
• zaczęła ≈ za-CHEW-wa
• klaskać ≈ KLAS-kach (with the final ć sounding like a soft ch)

A few useful notes:

• w in Polish sounds like English v
• ł sounds like English w
• cz sounds like ch in chop
• ć is a softer sound than English ch

If you say it slowly in chunks, it helps:

• wi-down-ia
• za-czę-ła
• klas-kać