Learn Hardcore Greek: Πριν να κοιμηθώ, κλείνω την κουρτίνα στο δωμάτιο για να μην μπαίνει φως. - Before I sleep, I close the curtain in the room so that no light comes in.

δεν: used with normal finite verbs in the indicative e.g. δεν κλείνω , δεν μπαίνει , δεν κοιμάμαι
μη(ν): used with subjunctive ( να -clauses), infinitive-like structures, and imperatives e.g. να μην μπαίνει , να μην φύγω , μη μιλάς

πριν + subjunctive (πριν κοιμηθώ) is slightly more common and a bit “cleaner” stylistically.
πριν να + subjunctive (πριν να κοιμηθώ) is also used a lot in everyday speech, and many Greeks say it naturally without thinking.

So:

You can drop να: Πριν κοιμηθώ, κλείνω την κουρτίνα…
Using να is not wrong; it’s just a variant. Some teachers prefer to teach πριν without να for simplicity and style.

Why is it κοιμηθώ and not κοιμάμαι?

Both verbs are related to sleeping, but they focus on different things:

Here we say πριν να κοιμηθώ = before I go to sleep, i.e. before the event or moment of falling asleep.

You wouldn’t normally say:

After πριν, when you mean “before I do X (once)”, you usually use the aorist subjunctive form, which here is κοιμηθώ.

Why is κλείνω in the present tense? In English I might say “Before I sleep, I / the curtain.”