Όταν προσγειωθεί το αεροπλάνο, μην σηκωθείς πριν από την ανακοίνωση.

After όταν for a specific future event, Greek normally uses the perfective non-past form, here προσγειωθεί.

So:

• Όταν προσγειωθεί το αεροπλάνο = When the plane lands / has landed
• Όταν προσγειώνεται το αεροπλάνο would sound more like whenever the plane lands or describe a repeated/habitual situation

Standard Greek usually does not use θα after όταν in this kind of sentence.

It is the 3rd person singular perfective non-past form of the verb προσγειώνομαι / προσγειώνω. In many learner grammars, you will see this described as an aorist subjunctive form.

In this sentence, it does not mean past tense. It is simply the form Greek uses here after όταν to refer to a one-time future action.

That is a very common point of confusion. In Greek, some verbs have middle/passive-looking forms but an active meaning.

So here:

• Το αεροπλάνο προσγειώνεται = The plane lands
• Όταν προσγειωθεί το αεροπλάνο = When the plane lands

It is not being used as a true passive here. Greek simply uses this verb in that form.

Because αεροπλάνο is a neuter singular noun, and the neuter singular definite article is το.

Also, το αεροπλάνο is the subject of προσγειωθεί. It is the thing that lands.

A detail that can confuse learners: for many neuter nouns, the nominative and accusative forms look the same. Here it is nominative, because it is the subject.

Because Greek normally does not use a negative imperative the way English does. Instead, negative commands are formed with μη(ν) + the appropriate verb form.

So:

• σηκώσου! = Get up!
• μην σηκωθείς! = Don’t get up!

This is one of the most important patterns to learn in Greek.

It comes from σηκώνομαι, which means to get up or to stand up.

In this airplane context, μην σηκωθείς means:

• don’t stand up
• more naturally, don’t get up from your seat

It does not mean wake up here.

This is an aspect difference.

• μην σηκωθείς is perfective: don’t get up (don’t do that action, even once)
• μην σηκώνεσαι is imperfective: don’t be getting up / don’t keep getting up / don’t get up habitually

In an airplane instruction, μην σηκωθείς is the natural choice, because it refers to one specific action.

πριν από means before when it is followed by a noun phrase.

Here, την ανακοίνωση is a noun phrase, so:

• πριν από την ανακοίνωση = before the announcement

Greek also uses πριν with a verb clause, for example:

• πριν γίνει η ανακοίνωση = before the announcement is made

So a helpful shortcut is:

• πριν από + noun
• πριν + clause/verb idea

Because ανακοίνωση is a feminine noun, and here it appears in the accusative singular, so the article is την.

It is accusative because it follows από, and από takes the accusative.

So:

• η ανακοίνωση = the announcement (nominative)
• την ανακοίνωση = the announcement (accusative)

English does not show this difference, but Greek does.

Yes. σηκωθείς is 2nd person singular, so it means you (singular, informal).

A real airline announcement to all passengers would more naturally use the plural/polite form:

• Όταν προσγειωθεί το αεροπλάνο, μην σηκωθείτε πριν από την ανακοίνωση.

That means the same thing, but addressed to a group.

Because Όταν προσγειωθεί το αεροπλάνο is an introductory time clause: When the plane lands.

Greek, like English, often separates that kind of clause with a comma when it comes first:

• Όταν προσγειωθεί το αεροπλάνο, μην σηκωθείς...

It helps mark the structure clearly:

1. When the plane lands
2. don’t get up before the announcement

Yes, Greek word order is fairly flexible, but the original version is the clearest and most natural.

The sentence begins with the time condition:

• Όταν προσγειωθεί το αεροπλάνο = When the plane lands

and then gives the instruction:

• μην σηκωθείς πριν από την ανακοίνωση = don’t get up before the announcement

That order makes very good sense, especially for instructions.