Nous prenons l'escalier de secours quand l'ascenseur est en panne.

Prenons is the present tense form of prendre for nous:

• je prends
• tu prends
• il/elle prend
• nous prenons
• vous prenez
• ils/elles prennent

So Nous prenons means we take or, more naturally here, we take/use the stairs.

In this sentence, it describes a habitual action: whenever the elevator is out of order, we take the emergency stairs.

Because prendre is an irregular verb.

In the nous and vous forms, the stem changes:

• nous prenons
• vous prenez

So even though many verbs use a regular pattern, prendre does not keep the d in these forms.

This is just something learners need to memorize as part of the conjugation.

Yes. French uses prendre in the same kind of way English does in expressions like:

• take the stairs
• take the elevator
• take the bus

So Nous prenons l'escalier means We take the stairs or We use the stairs.

It does not mean physically grabbing the staircase. It is a normal idiomatic use of prendre.

L'escalier de secours means the emergency staircase, the fire escape stairs, or the emergency stairs, depending on context.

Breakdown:

• escalier = staircase / stairs
• de secours = emergency / rescue / for emergencies

So the whole phrase refers to the stairs used in an emergency or when the normal route is not available.

French often uses escalier in the singular where English prefers stairs.

So:

• prendre l'escalier = take the stairs
• monter l'escalier = go up the stairs

French is thinking more in terms of the staircase as one structure, while English often uses the plural stairs.

So the singular here is completely normal.

Because de secours is a fixed-type construction in French.

Literally, it is something like staircase of emergency/help, but in natural English that becomes:

• emergency staircase
• fire escape stairs

French often uses noun + de + noun where English prefers an adjective:

• trousse de secours = first-aid kit
• sortie de secours = emergency exit
• escalier de secours = emergency staircase

So secours is not acting like a normal adjective here. It is part of a set phrase.

Because both escalier and ascenseur begin with a vowel sound.

In French, le or la becomes l' before a vowel or silent h. This is called elision.

So:

• le escalier → l'escalier
• le ascenseur → l'ascenseur

This makes pronunciation smoother.

Quand means when.

So:

• quand l'ascenseur est en panne = when the elevator is out of order

You would not use si here, because si usually means if, not when.

You could use lorsque, which also means when, and it is a bit more formal:

• Nous prenons l'escalier de secours lorsque l'ascenseur est en panne.

That is correct, but quand is very common and natural.

Être en panne is the standard expression for a machine, device, or system being out of order, broken down, or not working.

So:

• L'ascenseur est en panne = The elevator is out of order

You may also hear:

• La voiture est en panne = The car has broken down

Cassé means broken, but it often sounds more like something is physically broken. For machines and services, en panne is usually the more natural choice.

Because French, like English, often uses the present tense for habitual or general situations.

This sentence means something like:

• We take the emergency stairs whenever the elevator is out of order.

It is not necessarily happening right this second. It describes what usually happens under that condition.

So the present tense here expresses a regular pattern.

Yes, absolutely.

In everyday spoken French, on is very often used instead of nous:

• On prend l'escalier de secours quand l'ascenseur est en panne.

This still means we take the emergency stairs when the elevator is out of order.

Difference in tone:

• nous prenons = a bit more formal or written
• on prend = very common in conversation

Both are correct.

French uses definite articles very naturally in cases where English also often uses the, especially when talking about something understood from the situation.

Here, the sentence refers to:

• the emergency staircase
• the elevator

These are treated as known, identifiable things in the building or situation being discussed.

So l'escalier and l'ascenseur are completely normal here.