У нас в машине лежит запасная маска на случай дыма или пыли.

Russian often expresses possession with the pattern у + genitive + есть / location / object, rather than with a direct verb like English have.

So У нас в машине лежит запасная маска literally feels more like:

At us, in the car, lies a spare mask.

But in natural English, that usually becomes:

We have a spare mask in the car.

Here, нас is the genitive form of мы after the preposition у.

Лежит means is lying or lies. Russian often uses a specific verb of position when talking about where something is physically located.

So:

• лежит = is lying
• стоит = is standing
• висит = is hanging

In this sentence, the mask is imagined as physically lying in the car, so лежит is very natural.

If you used есть, that would only express existence, not position. In the present tense, есть is also often omitted in Russian. So лежит gives a more vivid and natural picture than just there is.

В машине means in the car.

After в, when you are talking about location, Russian uses the prepositional case:

• машина → в машине

Compare:

• в машине = in the car
• в доме = in the house
• в сумке = in the bag

If motion into something were meant, Russian would usually use the accusative instead:

• в машину = into the car

But here the mask is already located there, so в машине is correct.

Запасная маска is in the nominative singular feminine because it is the subject of the sentence.

• маска is feminine singular
• the adjective must agree with it
• so запасной changes to запасная

Agreement:

• masculine: запасной фильтр
• feminine: запасная маска
• neuter: запасное колесо
• plural: запасные части

So запасная маска means a spare mask.

Запасная means spare, extra, or kept in reserve.

It suggests that this is not necessarily the mask being used right now. It is one that is kept ready for a possible need.

Common examples:

• запасное колесо = spare tire
• запасной ключ = spare key
• запасная маска = spare mask

So the sentence suggests the mask is stored in the car just in case it becomes necessary.

На случай is a very common expression meaning in case of or for the possibility of.

In this sentence:

• на случай дыма или пыли = in case of smoke or dust

It introduces a possible situation that someone wants to be prepared for.

Other examples:

• на случай дождя = in case of rain
• на случай аварии = in case of an accident
• на случай пожара = in case of a fire

This is a fixed expression, and it is normally followed by the genitive case.

They are in the genitive case because на случай requires the genitive.

Base forms:

• дым → дыма
• пыль → пыли

So:

• на случай дыма
• на случай пыли

Since или connects two nouns here, both stay in the same case.

This is not because of или itself. It is because the whole phrase на случай governs the genitive.

Russian has no articles like English a or the.

So запасная маска can mean:

• a spare mask
• the spare mask

Which one is intended depends on context.

In this sentence, English naturally uses a spare mask, but Russian does not need any special word for that.

No, Russian word order is fairly flexible. The sentence can be rearranged to change emphasis.

Original:

У нас в машине лежит запасная маска на случай дыма или пыли.

Possible variations:

• В машине у нас лежит запасная маска...
  Emphasizes the location.
• Запасная маска лежит у нас в машине...
  Emphasizes the mask itself.
• На случай дыма или pыли у нас в машине лежит запасная маска.
  Emphasizes the purpose.

The original order sounds natural and neutral.

Yes, that is a more literal translation of the Russian structure.

But in normal English, We have a spare mask in the car is usually the more natural translation.

So there are really two levels:

• more literal: A spare mask is lying in our car in case of smoke or dust.
• more natural English: We have a spare mask in the car in case of smoke or dust.

Russian often uses location-based wording where English prefers have.

By itself, маска just means mask. The exact kind depends on context.

Because the sentence mentions дым and пыль, the listener will probably understand it as some kind of protective face mask or respirator-style mask, not a costume mask.

If the speaker wanted to be more specific, they might say something like:

• защитная маска = protective mask
• респиратор = respirator

But маска is enough if the context already makes the meaning clear.