Der Notausgang muss bis zur Landung geschlossen bleiben.

Because Notausgang is a masculine noun in German, so its nominative singular article is der.

• der Notausgang = the emergency exit
• grammatical gender in German does not always match any logical rule, so this is something you usually learn together with the noun

Also, in this sentence, der Notausgang is the subject, so the nominative form der is required.

Notausgang is a compound noun:

• Not = emergency, distress, need
• Ausgang = exit

So Notausgang literally means emergency exit.

German uses compound nouns very often, and the last part usually determines the gender and basic meaning. Since Ausgang is masculine, Notausgang is masculine too.

Because the subject is der Notausgang, which is third person singular.

The verb müssen means must / have to, and it changes form depending on the subject:

• ich muss
• du musst
• er/sie/es muss
• wir müssen
• ihr müsst
• sie/Sie müssen

Since der Notausgang is like it, German uses muss.

Because muss is a modal verb, and in a main clause German usually puts the modal verb in the normal conjugated position and sends the other verb to the end.

Here:

• muss = conjugated modal verb
• bleiben = second verb in the infinitive, so it goes to the end

Structure:

• Der Notausgang | muss | bis zur Landung | geschlossen bleiben

This is very common in German:

• Ich muss gehen.
• Die Tür muss offen bleiben.

It means remain closed or stay closed.

• geschlossen = closed
• bleiben = remain, stay

So geschlossen bleiben is a very common German pattern:

• Die Tür bleibt offen. = The door stays open.
• Das Fenster muss geschlossen bleiben. = The window must remain closed.

Here, geschlossen behaves like an adjective describing the state of the emergency exit.

This means until landing.

It breaks down like this:

• bis = until
• zur = contraction of zu der
• Landung = landing

So literally it is something like until to the landing, but in natural English that becomes until landing or until the landing.

A useful thing to know: with expressions like bis zu, the preposition zu determines the case, and zu takes the dative. That is why you get der Landung, which becomes zur Landung.

Because zur is simply the standard contraction of zu der.

• zu dem → zum
• zu der → zur

So:

• bis zu der Landung is grammatically possible
• bis zur Landung is the normal, natural form

German uses these contractions very often.

Because German often uses a noun where English might use a more verb-like expression.

• landen = to land
• die Landung = the landing

So bis zur Landung literally uses the noun landing.

This is very normal in German:

• vor der Abfahrt = before departure
• nach der Ankunft = after arrival
• während des Fluges = during the flight

Using Landung makes the phrase compact and natural.

In this sentence, it is best understood as an adjective describing a state: closed.

It comes historically from the past participle of schließen (to close), but in sentences like this it functions like an adjective:

• Die Tür ist geschlossen. = The door is closed.
• Die Tür muss geschlossen bleiben. = The door must remain closed.

So you do not need to think of it here as part of a perfect tense or passive form. It is describing the condition the emergency exit must stay in.

The sentence follows a very typical German main-clause pattern:

1. Subject: Der Notausgang
2. Finite verb: muss
3. Other information: bis zur Landung
4. Infinitive at the end: bleiben

And geschlossen sits with bleiben because they form the idea remain closed.

So the structure is:

• Der Notausgang = subject
• muss = finite verb in second position
• bis zur Landung = time expression
• geschlossen bleiben = verbal idea at the end

German often puts the most important verb material at the end when a modal verb is present.

Yes, that is possible, but it is slightly different in feel.

• geschlossen bleiben = remain closed, stay closed
• geschlossen sein = be closed

In this context:

• muss geschlossen bleiben strongly emphasizes that it should continue in that closed state
• muss geschlossen sein emphasizes the required state itself: it must be closed

Both can work, but geschlossen bleiben is especially natural when the idea is do not open it before landing.

German often expresses this idea more directly with geschlossen bleiben rather than a more passive-looking structure.

English might say:

• must remain closed
• must stay closed
• must be kept closed

German commonly says:

• muss geschlossen bleiben

It is simple, idiomatic, and very common in instructions, signs, and safety announcements.