При пожаре нельзя пользоваться лифтом.

Because при requires the prepositional case in this meaning.

• пожар = fire
• при пожаре = in case of fire / during a fire

So:

• nominative: пожар
• prepositional: при пожаре

This is a fixed and very common pattern: при + prepositional.

Here при means something like in the event of, in case of, or during.

So при пожаре is not literally just near the fire. In this kind of safety language, it means:

• if there is a fire
• in case of fire

That is why this phrase is common on warning signs and instructions.

Нельзя means it is not allowed, one must not, or you cannot in the sense of prohibition.

In this sentence, it is a strong warning:

• При пожаре нельзя пользоваться лифтом.
• In case of fire, you must not use the elevator.

So this is not just about physical impossibility. It usually expresses a rule or prohibition.

Russian often uses impersonal constructions, especially for rules, instructions, and general statements.

Here:

• нельзя пользоваться лифтом = it is forbidden to use the elevator

There is no need to say you. The sentence applies generally to anyone.

English often uses you must not, but Russian can simply use:

• нельзя + infinitive

Both can relate to using, but пользоваться is especially common when talking about using a service, tool, device, or facility.

• пользоваться лифтом = to use the elevator
• пользоваться телефоном = to use the phone

Использовать often sounds more like to utilize something, sometimes a bit more formal or technical.

For a public safety sign, пользоваться лифтом is the most natural standard phrasing.

Because the verb пользоваться requires the instrumental case.

So:

• лифт = elevator
• лифтом = by means of the elevator / the elevator (instrumental)

This is just how the verb works:

• пользоваться чем? = to use what?
• пользоваться лифтом
• пользоваться телефоном
• пользоваться компьютером

So you should learn пользоваться together with the instrumental case.

Yes, very natural. This is exactly the kind of wording you might see on signs in Russian-speaking places.

It is concise, formal, and standard for safety instructions.

A similar sign might also say:

• Во время пожара не пользуйтесь лифтом.

That version is a direct command to a group or to the public. But your sentence is a general rule stated impersonally, which is very common on signs.

Both can mean do not use the elevator, but the style is different.

• нельзя пользоваться лифтом = it is forbidden to use the elevator

  • impersonal
  • sounds like a general rule or posted instruction

• не пользуйтесь лифтом = do not use the elevator

  • direct imperative
  • sounds like someone is directly telling people what to do

On warning signs, both are possible, but нельзя + infinitive often feels especially like a formal regulation.

Yes, you could. Both are understandable, but they are slightly different in tone.

• при пожаре = in case of fire / in a fire situation
• во время пожара = during the fire

For warnings and safety instructions, при пожаре is especially common and compact. It sounds like standard sign language.

So:

• При пожаре нельзя пользоваться лифтом.
  is the most typical sign-style phrasing.

Yes, Russian word order is fairly flexible, but this version is the most natural for a sign:

• При пожаре нельзя пользоваться лифтом.

It starts with the situation first, then the rule.

Other orders are possible, but they may sound less neutral or less sign-like. For example:

• Лифтом нельзя пользоваться при пожаре.

This is also correct, but it puts more focus on лифтом.

For a standard safety notice, your original sentence is the best choice.

A helpful stress guide is:

• При пожа́ре нельзя́ по́льзоваться ли́фтом.

Approximate pronunciation:

• pree pa-ZHA-rye nye-LZYA POL-zuh-vat-sya LEEF-tam

A few important points:

• нельзя has stress on the last syllable: нельзя́
• пожаре has stress on жа́
• лифтом has stress on ли́ф

Also note that ться in пользоваться is pronounced roughly like -tsa, not like English t + s + ya separately.

This sentence is a great example of two important patterns:

1. при + prepositional

   • при пожаре

2. пользоваться + instrumental

   • пользоваться лифтом

So if you want to build similar sentences, you can use the same structure:

• При аварии нельзя пользоваться телефоном.
• При поломке нельзя пользоваться устройством.

The key grammar takeaway is:

• при → prepositional
• пользоваться → instrumental