Τη στιγμή που ανοίγει η αυλαία, μπαίνει ο συμπρωταγωνιστής.

The moment the curtain opens, the co-star enters.

Breakdown of Τη στιγμή που ανοίγει η αυλαία, μπαίνει ο συμπρωταγωνιστής.

• ανοίγω: to open
• μπαίνω: to enter
• τη στιγμή που: the moment that
• ο συμπρωταγωνιστής: the co-star

Questions & Answers about Τη στιγμή που ανοίγει η αυλαία, μπαίνει ο συμπρωταγωνιστής.

Why is it τη στιγμή and not η στιγμή or την στιγμή?

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.

What does που mean 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.