Τη στιγμή που ανοίγει η αυλαία, μπαίνει ο συμπρωταγωνιστής.

The dictionary form is η στιγμή = the moment.

In this sentence, though, τη στιγμή που... is a fixed time expression meaning the moment that / when..., so στιγμή appears in the accusative: τη στιγμή.

As for τη vs την:

• the full accusative article is την
• in Modern Greek, the final -ν is often dropped before many consonants
• since στιγμή begins with στ-, τη στιγμή is the normal form

So:

• η στιγμή = nominative, dictionary form
• τη στιγμή = accusative, used here in a time expression

You may still sometimes see την στιγμή, but τη στιγμή is the usual spelling here.

Here που means something like when or that.

So τη στιγμή που ανοίγει η αυλαία is literally:

• the moment that the curtain opens

Natural English would usually say:

• the moment the curtain opens
• when the curtain opens

This is a very common use of που in Greek. It is not always where.

Yes. Όταν also means when, so this would be correct:

Όταν ανοίγει η αυλαία, μπαίνει ο συμπρωταγωνιστής.

The difference is mainly one of nuance:

• όταν = plain when
• τη στιγμή που = the moment when / at the very moment that

So τη στιγμή που sounds a bit more precise or dramatic.

Because Greek often uses the present tense to describe:

• actions happening in front of us
• stage directions
• plot summary
• vivid narration
• repeated or typical actions

This sentence sounds like the speaker is describing what happens on stage or in a scene:

• the curtain opens
• the co-star enters

So the present tense feels natural.

If you were talking about one completed event in the past, you would normally switch to past forms, for example:

Τη στιγμή που άνοιξε η αυλαία, μπήκε ο συμπρωταγωνιστής.

Because Greek word order is more flexible than English.

Both of these are possible:

• Ανοίγει η αυλαία
• Η αυλαία ανοίγει

and

• Μπαίνει ο συμπρωταγωνιστής
• Ο συμπρωταγωνιστής μπαίνει

In your sentence, the verb-first order makes the actions feel more immediate, almost like stage directions:

• opens the curtain
• enters the co-star

Of course, natural English cannot copy that word order directly, but in Greek it is completely normal.

Here it means the curtain opens.

The clue is the case:

• η αυλαία is nominative, so it is the subject
• therefore, the curtain is the thing doing the action grammatically

If you wanted to say someone opens the curtain, you would use the curtain as an object:

• ανοίγει την αυλαία = he/she opens the curtain

So:

• ανοίγει η αυλαία = the curtain opens
• ανοίγει την αυλαία = he/she opens the curtain

Μπαίνει is the 3rd person singular present of μπαίνω, which means:

• to enter
• to go in
• to come in

So μπαίνει ο συμπρωταγωνιστής means:

• the co-star enters
• the co-star comes in

Μπαίνω is a very common everyday verb. It is much more ordinary and conversational than a more formal verb like εισέρχομαι.

Because Greek uses the definite article more often than English.

Here ο συμπρωταγωνιστής means the co-star, referring to a specific, identifiable person in the scene.

In Greek, it is very natural to include the article with nouns like this, especially when talking about a known person, role, or thing.

So even if English sometimes sounds a bit lighter without an article in certain contexts, Greek usually keeps it:

• ο συμπρωταγωνιστής
• η αυλαία

Συμπρωταγωνιστής means co-star or co-lead.

It is built from:

• συν-/συμ- = with, co-
• πρωταγωνιστής = lead actor / protagonist

So συμπρωταγωνιστής is someone who shares the leading role, not just any minor supporting actor.

Depending on context, English could translate it as:

• co-star
• co-lead
• fellow lead actor

The comma separates the introductory time clause from the main clause:

• Τη στιγμή που ανοίγει η αυλαία = time clause
• μπαίνει ο συμπρωταγωνιστής = main clause

So the comma works much like it does in English after an opening clause:

• When the curtain opens, the co-star enters.

It helps the sentence read more clearly.